Expressions

In general, any mathematical expression accepted by C, FORTRAN, Pascal, or BASIC is valid. The precedence of these operators is determined by the specifications of the C programming language. White space (spaces and tabs) is ignored inside expressions.

Complex constants are expressed as {<real>,<imag>}, where <real> and <imag> must be numerical constants. For example, {3,2} represents 3 + 2i; {0,1} represents 'i' itself. The curly braces are explicitly required here.

Note that gnuplot uses both "real" and "integer" arithmetic, like FORTRAN and C. Integers are entered as "1", "-10", etc; reals as "1.0", "-10.0", "1e1", 3.5e-1, etc. The most important difference between the two forms is in division: division of integers truncates: 5/2 = 2; division of reals does not: 5.0/2.0 = 2.5. In mixed expressions, integers are "promoted" to reals before evaluation: 5/2e0 = 2.5. The result of division of a negative integer by a positive one may vary among compilers. Try a test like "print -5/2" to determine if your system chooses -2 or -3 as the answer.

The integer expression "1/0" may be used to generate an "undefined" flag, which causes a point to ignored. Or you can use the pre-defined variable NaN to achieve the same result. See using for an example.

The real and imaginary parts of complex expressions are always real, whatever the form in which they are entered: in {3,2} the "3" and "2" are reals, not integers.

Gnuplot can also perform simple operations on strings and string variables. For example, the expression ("A" . "B" eq "AB") evaluates as true, illustrating the string concatenation operator and the string equality operator.

A string which contains a numerical value is promoted to the corresponding integer or real value if used in a numerical expression. Thus ("3" + "4" == 7) and (6.78 == "6.78") both evaluate to true. An integer, but not a real or complex value, is promoted to a string if used in string concatenation. A typical case is the use of integers to construct file names or other strings; e.g. ("file" . 4 eq "file4") is true.

Substrings can be specified using a postfixed range descriptor [beg:end]. For example, "ABCDEF"[3:4] == "CD" and "ABCDEF"[4:*] == "DEF" The syntax "string"[beg:end] is exactly equivalent to calling the built-in string-valued function substr("string",beg,end), except that you cannot omit either beg or end from the function call.

Functions

Arguments to math functions in gnuplot can be integer, real, or complex unless otherwise noted. Functions that accept or return angles (e.g. sin(x)) treat angle values as radians, but this may be changed to degrees using the command set angles.

Math library functions
Function Arguments Returns
abs(x) any |x|, absolute value of x; same type
abs(x) complex length of x, √( Re(x)2 + Im(x)2 )
acos(x) any cos-1 x (inverse cosine)
acosh(x) any cosh-1 x (inverse hyperbolic cosine) in radians
airy(x) any Airy function Ai(x)
arg(x) complex the phase of x
asin(x) any sin-1 x (inverse sin)
asinh(x) any sinh-1 x (inverse hyperbolic sin) in radians
atan(x) any tan-1 x (inverse tangent)
atan2(y,x) int or real tan-1(y/x) (inverse tangent)
atanh(x) any tanh-1 x (inverse hyperbolic tangent) in radians
EllipticK(k) real k in (-1:1) K(k) complete elliptic integral of the first kind
EllipticE(k) real k in [-1:1] E(k) complete elliptic integral of the second kind
EllipticPi(n,k) real n<1, real k in (-1:1) Π(n,k) complete elliptic integral of the third kind
besj0(x) int or real J0 Bessel function of x, in radians
besj1(x) int or real J1 Bessel function of x, in radians
besy0(x) int or real Y0 Bessel function of x, in radians
besy1(x) int or real Y1 Bessel function of x, in radians
ceil(x) any x⌉, smallest integer not less than x (real part)
cos(x) radians cos x, cosine of x
cosh(x) any cosh x, hyperbolic cosine of x in radians
erf(x) any erf(Re(x)), error function of real(x)
erfc(x) any erfc(Re(x)), 1.0 - error function of real(x)
exp(x) any ex, exponential function of x
expint(n,x) any En(x), exponential integral function of x
floor(x) any x⌋, largest integer not greater than x (real part)
gamma(x) any Γ(Re(x)), gamma function of real(x)
ibeta(p,q,x) any ibeta(Re(p,q,x)), ibeta function of real(p,q,x)
inverf(x) any inverse error function real(x)
igamma(a,x) any igamma(Re(a,x)), igamma function of real(a,x)
imag(x) complex Im(x), imaginary part of x as a real number
invnorm(x) any inverse normal distribution function real(x)
int(x) real integer part of x, truncated toward zero
lambertw(x) real Lambert W function
lgamma(x) any lgamma(Re(x)), lgamma function of real(x)
log(x) any ln x, natural logarithm (base e) of x
log10(x) any log10 x, logarithm (base 10) of x
norm(x) any norm(x), normal distribution function of real(x)
rand(x) int pseudo random number in the interval [0:1]
real(x) any Re(x), real part of x
sgn(x) any 1 if x > 0, -1 if x < 0, 0 if x = 0. ℑ(x) ignored
sin(x) any sin x, sine of x
sinh(x) any sinh x, hyperbolic sine of x in radians
sqrt(x) any x, square root of x
tan(x) any tan x, tangent of x
tanh(x) any tanh x, hyperbolic tangent of x in radians
voigt(x,y) real convolution of Gaussian and Lorentzian

 

Special functions from libcerf (only if available)
Function Arguments Returns
cerf(z) complex complex error function
cdawson(z) complex complex Dawson's integral
faddeeva(z) complex rescaled complex error function w(z) = exp(-z²) × erfc(-iz)
erfi(x) real imaginary error function erfi(x) = -i × erf(ix)
VP(x,sigma,gamma) real Voigt profile

 

String functions
Function Arguments Returns
gprintf("format",x,...) any string result from applying gnuplot's format parser
sprintf("format",x,...) multiple string result from C-language sprintf
strlen("string") string int length of string in bytes
strstrt("string","key") strings int index of first character of substring "key"
substr("string",beg,end) multiple string "string"[beg:end]
strftime("timeformat",t) any string result from applying gnuplot's time parser
strptime("timeformat",s) string seconds since year 1970 as given in string s
system("command") string string containing output stream of shell command
word("string",n) string, int returns the nth word in "string"
words("string") string returns the number of words in "string"

 

other gnuplot functions
Function Arguments Returns
column(x) int or string contents of column x during data input.
columnhead(x) int string containing first entry of column x in datafile.
exists("X") string returns 1 if a variable named X is defined, 0 otherwise.
hsv2rgb(h,s,v) h,s,v in [0:1] converts HSV color to 24bit RGB color.
stringcolumn(x) int content column x as a string.
timecolumn(N,format) int, string time data in column N during data input
tm_hour(x) int the hour
tm_mday(x) int the day of the month
tm_min(x) int the minute
tm_mon(x) int the month
tm_sec(x) int the second
tm_wday(x) int the day of the week
tm_yday(x) int the day of the year
tm_year(x) int the year
time(x) any the current system time
valid(x) int test validity of column(x) during datafile manip.
value("name") string returns the current value of the named variable.

Abs

The abs(x) function returns the absolute value of its argument. The returned value is of the same type as the argument.

For complex arguments, abs(x) is defined as the length of x in the complex plane [i.e., sqrt(real(x)**2 + imag(x)**2) ]. This is also known as the norm or complex modulus of x.

Acos

The acos(x) function returns the arc cosine (inverse cosine) of its argument. acos returns its argument in radians or degrees, as selected by set angles.

Acosh

The acosh(x) function returns the inverse hyperbolic cosine of its argument in radians.

Airy

The airy(x) function returns the value of the Airy function Ai(x) of its argument. The function Ai(x) is that solution of the equation y'' - x y = 0 which is everywhere finite. If the argument is complex, its imaginary part is ignored.

Arg

The arg(x) function returns the phase of a complex number in radians or degrees, as selected by set angles.

Asin

The asin(x) function returns the arc sin (inverse sin) of its argument. asin returns its argument in radians or degrees, as selected by set angles.

Asinh

The asinh(x) function returns the inverse hyperbolic sin of its argument in radians.

Atan

The atan(x) function returns the arc tangent (inverse tangent) of its argument. atan returns its argument in radians or degrees, as selected by set angles.

Atan2

The atan2(y,x) function returns the arc tangent (inverse tangent) of the ratio of the real parts of its arguments. atan2 returns its argument in radians or degrees, as selected by set angles, in the correct quadrant.

Atanh

The atanh(x) function returns the inverse hyperbolic tangent of its argument in radians.

Elliptick

See elliptic integrals.

Elliptice

See elliptic integrals.

Ellipticpi

See elliptic integrals.

Besj0

The besj0(x) function returns the J0th Bessel function of its argument. besj0 expects its argument to be in radians.

Besj1

The besj1(x) function returns the J1st Bessel function of its argument. besj1 expects its argument to be in radians.

Besy0

The besy0(x) function returns the Y0th Bessel function of its argument. besy0 expects its argument to be in radians.

Besy1

The besy1(x) function returns the Y1st Bessel function of its argument. besy1 expects its argument to be in radians.

Ceil

The ceil(x) function returns the smallest integer that is not less than its argument. For complex numbers, ceil returns the smallest integer not less than the real part of its argument.

Cos

The cos(x) function returns the cosine of its argument. cos accepts its argument in radians or degrees, as selected by set angles.

Cosh

The cosh(x) function returns the hyperbolic cosine of its argument. cosh expects its argument to be in radians.

Erf

The erf(x) function returns the error function of the real part of its argument. If the argument is a complex value, the imaginary component is ignored. See erfc, inverf, and norm.

Erfc

The erfc(x) function returns 1.0 - the error function of the real part of its argument. If the argument is a complex value, the imaginary component is ignored. See erf, inverf, and norm.

Exp

The exp(x) function returns the exponential function of its argument (e raised to the power of its argument). On some implementations (notably suns), exp(-x) returns undefined for very large x. A user-defined function like safe(x) = x<-100 ? 0 : exp(x) might prove useful in these cases.

Expint

The expint(n,x) function returns the exponential integral of the real part of its argument: integral from 1 to infinity of t^(-n) e^(-tx) dt. n must be a nonnegative integer, x>=0, and either x>0 or n>1.

Floor

The floor(x) function returns the largest integer not greater than its argument. For complex numbers, floor returns the largest integer not greater than the real part of its argument.

Gamma

The gamma(x) function returns the gamma function of the real part of its argument. For integer n, gamma(n+1) = n!. If the argument is a complex value, the imaginary component is ignored.

Ibeta

The ibeta(p,q,x) function returns the incomplete beta function of the real parts of its arguments. p, q > 0 and x in [0:1]. If the arguments are complex, the imaginary components are ignored. The function is approximated by the method of continued fractions (Abramowitz and Stegun, 1964). The approximation is only accurate in the region x < (p-1)/(p+q-2).

Inverf

The inverf(x) function returns the inverse error function of the real part of its argument. See erf and invnorm.

Igamma

The igamma(a,x) function returns the normalized incomplete gamma function of the real parts of its arguments, where a > 0 and x >= 0. The standard notation is P(a,x), e.g. Abramowitz and Stegun (6.5.1), with limiting value of 1 as x approaches infinity. If the arguments are complex, the imaginary components are ignored.

Imag

The imag(x) function returns the imaginary part of its argument as a real number.

Invnorm

The invnorm(x) function returns the inverse cumulative normal (Gaussian) distribution function of the real part of its argument. See norm.

Int

The int(x) function returns the integer part of its argument, truncated toward zero.

Lambertw

The lambertw function returns the value of the principal branch of Lambert's W function, which is defined by the equation (W(z)*exp(W(z))=z. z must be a real number with z >= -exp(-1).

Lgamma

The lgamma(x) function returns the natural logarithm of the gamma function of the real part of its argument. If the argument is a complex value, the imaginary component is ignored.

Log

The log(x) function returns the natural logarithm (base e) of its argument. See log10.

Log10

The log10(x) function returns the logarithm (base 10) of its argument.

Norm

The norm(x) function returns the cumulative normal (Gaussian) distribution function of the real part of its argument. See invnorm, erf and erfc.

Rand

rand(0) returns a pseudo random number in the interval [0:1]. See random for more details.

Real

The real(x) function returns the real part of its argument.

Sgn

The sgn(x) function returns 1 if its argument is positive, -1 if its argument is negative, and 0 if its argument is 0. If the argument is a complex value, the imaginary component is ignored.

Sin

The sin(x) function returns the sine of its argument. sin expects its argument to be in radians or degrees, as selected by set angles.

Sinh

The sinh(x) function returns the hyperbolic sine of its argument. sinh expects its argument to be in radians.

Sqrt

The sqrt(x) function returns the square root of its argument.

Tan

The tan(x) function returns the tangent of its argument. tan expects its argument to be in radians or degrees, as selected by set angles.

Tanh

The tanh(x) function returns the hyperbolic tangent of its argument. tanh expects its argument to be in radians.

Voigt

The function voigt(x,y) returns an approximation to the Voigt/Faddeeva function used in spectral analysis. The approximation is accurate to one part in 10^4. If the libcerf routines are available, the re_w_of_z() routine is used to provide a more accurate value. Note that voigt(x,y) = real(faddeeva( x + y*{0,1} )).

Cerf

cerf(z) is the complex version of the error function erf(x)

Cdawson

cdawson(z) returns Dawson's Integral evaluated for the complex argument z. cdawson(z) = sqrt(pi)/2 * exp(-z^2) * erfi(z)

Faddeeva

Faddeeva(z) returns the rescaled complex error function

This corresponds to Eqs 7.1.3 and 7.1.4 of Abramowitz and Stegun.

Erfi

Imaginary error function erfi(x) = -i * erf(ix)

Voigt profile

VP(x,sigma,gamma) corresponds to the Voigt profile defined by convolution of a Gaussian G(x;sigma) with a Lorentzian L(x;gamma).

Gprintf

gprintf("format",x) applies gnuplot's own format specifiers to the single variable x and returns the resulting string. If you want standard C-language format specifiers, you must instead use sprintf("format",x). See format specifiers.

Sprintf

sprintf("format",var1,var2,...) applies standard C-language format specifiers to multiple arguments and returns the resulting string. If you want to use gnuplot's own format specifiers, you must instead call gprintf(). For information on sprintf format specifiers, please see standard C-language documentation or the unix sprintf man page.

Strlen

strlen("string") returns the length of the string in bytes. If the current encoding supports multibyte characters, this may be larger than the number of characters in the string.

Strstrt

strstrt("string","key") searches for the character string "key" in "string" and returns the index to the first character of "key". If "key" is not found, returns 0. Similar to C library function strstr except that it returns an index rather than a string pointer. strstrt("hayneedlestack","needle") = 4.

Substr

substr("string",beg,end) returns the substring consisting of characters beg through end of the original string. This is exactly equivalent to the expression "string"[beg:end] except that you do not have the option of omitting beg or end.

Strftime

strftime("timeformat",t) applies the timeformat specifiers to the time t given in seconds since the year 1970. See time_specifiers and strptime.

Strptime

strptime("timeformat",s) reads the time from the string s using the timeformat specifiers and converts it into seconds since the year 1970. See time_specifiers and strftime.

System

system("command") executes "command" using the standard shell and returns the resulting character stream from stdout as string variable. One optional trailing newline is ignored.

This can be used to import external functions into gnuplot scripts using 'f(x) = real(system(sprintf("somecommand %f", x)))'.

Word

word("string",n) returns the nth word in string. For example, word("one two three",2) returns the string "two".

Words

words("string") returns the number of words in string. For example, words(" a b c d") returns 4.

Column

column(x) may be used only in expressions as part of using manipulations to fits or datafile plots. It evaluates to the numerical value of the contents of column x. See plot datafile using.

Columnhead

columnhead(x) may only be used in expressions as part of using manipulations to fits or datafile plots. It evaluates to a string containing the contents of column x in the first line of data. See plot datafile using.

Exists

The argument to exists() is a string constant or a string variable; if the string contains the name of a defined variable, the function returns 1. Otherwise the function returns 0.

Hsv2rgb

The HSV (Hue/Saturation/Value) triplet is converted to an equivalent RGB value.

Stringcolumn

stringcolumn(x) may be used only in expressions as part of using manipulations to fits or datafile plots. It returns the content of column x as a string variable. See plot datafile using.

Timecolumn

timecolumn(N,"timeformat") may be used only in expressions as part of using manipulations to fits or datafile plots. See plot datafile using.

It reads string data starting at column N as a time/date value and uses "timeformat" to interpret this as "seconds since the epoch" to millisecond precision. Note: prior to version 5 this function took only a single parameter and worked only for columns that contained purely an axis coordinate.

Tm_hour

The tm_hour function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the hour (an integer in the range 0--23) as a real.

Tm_mday

The tm_mday function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the day of the month (an integer in the range 1--31) as a real.

Tm_min

The tm_min function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the minute (an integer in the range 0--59) as a real.

Tm_mon

The tm_mon function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the month (an integer in the range 0--11) as a real.

Tm_sec

The tm_sec function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the second (an integer in the range 0--59) as a real.

Tm_wday

The tm_wday function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the day of the week (an integer in the range 0--6) as a real.

Tm_yday

The tm_yday function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the day of the year (an integer in the range 1--366) as a real.

Tm_year

The tm_year function interprets its argument as a time, in seconds from 1 Jan 1970. It returns the year (an integer) as a real.

Time

The time function returns the current system time. This value can be converted to a date string with the strftime function, or it can be used in conjunction with timecolumn to generate relative time/date plots. The type of the argument determines what is returned. If the argument is an integer, time() returns the current time as an integer, in seconds from 1 Jan 1970. If the argument is real (or complex), the result is real as well. If the argument is a string, it is assumed to be a format string, and it is passed to strftime to provide a formatted time string.

Valid

valid(x) may be used only in expressions as part of using manipulations to fits or datafile plots. See plot datafile using.

Elliptic integrals

The EllipticK(k) function returns the complete elliptic integral of the first kind, i.e. the definite integral between 0 and pi/2 of the function (1-(k*sin(p))**2)**(-0.5). The domain of k is -1 to 1 (exclusive).

The EllipticE(k) function returns the complete elliptic integral of the second kind, i.e. the definite integral between 0 and pi/2 of the function (1-(k*sin(p))**2)**0.5. The domain of k is -1 to 1 (inclusive).

The EllipticPi(n,k) function returns the complete elliptic integral of the third kind, i.e. the definite integral between 0 and pi/2 of the function (1-(k*sin(p))**2)**(-0.5)/(1-n*sin(p)**2). The parameter n must be less than 1, while k must lie between -1 and 1 (exclusive). Note that by definition EllipticPi(0,k) == EllipticK(k) for all possible values of k.

Random number generator

The function rand() produces a sequence of pseudo-random numbers between 0 and 1 using an algorithm from P. L'Ecuyer and S. Cote, "Implementing a random number package with splitting facilities", ACM Transactions on Mathematical Software, 17:98-111 (1991).

rand(0)     returns a pseudo random number in the interval [0:1]
            generated from the current value of two internal
            32-bit seeds.
rand(-1)    resets both seeds to a standard value.
rand(x)     for integer 0 < x < 2^31-1 sets both internal seeds
            to x.
rand({x,y}) for integer 0 < x,y < 2^31-1 sets seed1 to x and 
            seed2 to y.

Value

B = value("A") is effectively the same as B = A, where A is the name of a user-defined variable. This is useful when the name of the variable is itself held in a string variable. See user-defined variables. It also allows you to read the name of a variable from a data file. If the argument is a numerical expression, value() returns the value of that expression. If the argument is a string that does not correspond to a currently defined variable, value() returns NaN.

Operators

The operators in gnuplot are the same as the corresponding operators in the C programming language, except that all operators accept integer, real, and complex arguments, unless otherwise noted. The ** operator (exponentiation) is supported, as in FORTRAN.

Parentheses may be used to change order of evaluation.

Unary

The following is a list of all the unary operators and their usages:

Symbol Example Explanation
- -a unary minus
+ +a unary plus (no-operation)
~ ~a * one's complement
! !a * logical negation
! a! * factorial
$ $3 * call arg/column during `using` manipulation

(*) Starred explanations indicate that the operator requires an integer argument.

Operator precedence is the same as in Fortran and C. As in those languages, parentheses may be used to change the order of operation. Thus -2**2 = -4, but (-2)**2 = 4.

The factorial operator returns a real number to allow a greater range.

Binary

The following is a list of all the binary operators and their usages:

Symbol Example Explanation
** a**b exponentiation
* a*b multiplication
/ a/b division
% a%b * modulo
+ a+b addition
- a-b subtraction
== a==b equality
!= a!=b inequality
< a<b less than
<= a<=b less than or equal to
> a>b greater than
>= a>=b greater than or equal to
<< 0xff<<1 left shift unsigned
>> 0xff>>1 right shift unsigned
& a&b * bitwise AND
^ a^b * bitwise exclusive OR
| a|b * bitwise inclusive OR
&& a&&b * logical AND
|| a||b * logical OR
= a = b assignment
, (a,b) serial evaluation
. a.b string concatenation
eq A eq B string equality
ne A ne B string inequality

(*) Starred explanations indicate that the operator requires integer arguments. Capital letters A and B indicate that the operator requires string arguments.

Logical AND (&&) and OR (||) short-circuit the way they do in C. That is, the second && operand is not evaluated if the first is false; the second || operand is not evaluated if the first is true.

Serial evaluation occurs only in parentheses and is guaranteed to proceed in left to right order. The value of the rightmost subexpression is returned.

Ternary

There is a single ternary operator:

Symbol Example Explanation
?: a?b:c * ternary operation

The ternary operator behaves as it does in C. The first argument (a), which must be an integer, is evaluated. If it is true (non-zero), the second argument (b) is evaluated and returned; otherwise the third argument (c) is evaluated and returned.

The ternary operator is very useful both in constructing piecewise functions and in plotting points only when certain conditions are met.

Examples:

Plot a function that is to equal sin(x) for 0 <= x < 1, 1/x for 1 <= x < 2, and undefined elsewhere:

f(x) = 0<=x && x<1 ? sin(x) : 1<=x && x<2 ? 1/x : 1/0
plot f(x)

Note that gnuplot quietly ignores undefined values, so the final branch of the function (1/0) will produce no plottable points. Note also that f(x) will be plotted as a continuous function across the discontinuity if a line style is used. To plot it discontinuously, create separate functions for the two pieces. (Parametric functions are also useful for this purpose.)

For data in a file, plot the average of the data in columns 2 and 3 against the datum in column 1, but only if the datum in column 4 is non-negative:

plot 'file' using 1:( $4<0 ? 1/0 : ($2+$3)/2 )

For an explanation of the using syntax, please see plot datafile using.

Summation

A summation expression has the form

sum [<var> = <start> : <end>] <expression>

<var> is treated as an integer variable that takes on successive integral values from <start> to <end>. For each of these, the current value of <expression> is added to a running total whose final value becomes the value of the summation expression. Examples:

print sum [i=1:10] i
    55.
# Equivalent to plot 'data' using 1:($2+$3+$4+$5+$6+...)
plot 'data' using 1 : (sum [col=2:MAXCOL] column(col))

It is not necessary that <expression> contain the variable <var>. Although <start> and <end> can be specified as variables or expressions, their value cannot be changed dynamically as a side-effect of carrying out the summation. If <end> is less than <start> then the value of the summation is zero.

Gnuplot-defined variables

Gnuplot maintains a number of read-only variables that reflect the current internal state of the program and the most recent plot. These variables begin with the prefix "GPVAL_". Examples include GPVAL_TERM, GPVAL_X_MIN, GPVAL_X_MAX, GPVAL_Y_MIN. Type show variables all to display the complete list and current values. Values related to axes parameters (ranges, log base) are values used during the last plot, not those currently set.

Example: To calculate the fractional screen coordinates of the point [X,Y]

GRAPH_X = (X - GPVAL_X_MIN) / (GPVAL_X_MAX - GPVAL_X_MIN)
GRAPH_Y = (Y - GPVAL_Y_MIN) / (GPVAL_Y_MAX - GPVAL_Y_MIN)
SCREEN_X = GPVAL_TERM_XMIN + GRAPH_X * (GPVAL_TERM_XMAX - GPVAL_TERM_XMIN) 
SCREEN_Y = GPVAL_TERM_YMIN + GRAPH_Y * (GPVAL_TERM_YMAX - GPVAL_TERM_YMIN)
FRAC_X = SCREEN_X / GPVAL_TERM_XSIZE
FRAC_Y = SCREEN_Y / GPVAL_TERM_YSIZE

The read-only variable GPVAL_ERRNO is set to a non-zero value if any gnuplot command terminates early due to an error. The most recent error message is stored in the string variable GPVAL_ERRMSG. Both GPVAL_ERRNO and GPVAL_ERRMSG can be cleared using the command reset errors.

Interactive terminals with mouse functionality maintain read-only variables with the prefix "MOUSE_". See mouse variables for details.

The fit mechanism uses several variables with names that begin "FIT_". It is safest to avoid using such names. When using set fit errorvariables, the error for each fitted parameter will be stored in a variable named like the parameter, but with "_err" appended. See the documentation on fit and set fit for details.

See user-defined variables, reset errors, mouse variables, and fit.

User-defined variables and functions

New user-defined variables and functions of one through twelve variables may be declared and used anywhere, including on the plot command itself.

User-defined function syntax:

<func-name>( <dummy1> {,<dummy2>} ... {,<dummy12>} ) = <expression>

where <expression> is defined in terms of <dummy1> through <dummy12>.

User-defined variable syntax:

<variable-name> = <constant-expression>

Examples:

w = 2
q = floor(tan(pi/2 - 0.1))
f(x) = sin(w*x)
sinc(x) = sin(pi*x)/(pi*x)
delta(t) = (t == 0)
ramp(t) = (t > 0) ? t : 0
min(a,b) = (a < b) ? a : b
comb(n,k) = n!/(k!*(n-k)!)
len3d(x,y,z) = sqrt(x*x+y*y+z*z)
plot f(x) = sin(x*a), a = 0.2, f(x), a = 0.4, f(x)
file = "mydata.inp"
file(n) = sprintf("run_%d.dat",n)

The final two examples illustrate a user-defined string variable and a user-defined string function.

Note that the variables pi (3.14159...) and NaN (IEEE "Not a Number") are already defined. You can redefine these to something else if you really need to. The original values can be recovered by setting:

NaN = GPVAL_NaN
pi  = GPVAL_pi

Other variables may be defined under various gnuplot operations like mousing in interactive terminals or fitting; see gnuplot-defined variables for details.

You can check for existence of a given variable V by the exists("V") expression. For example

a = 10
if (exists("a")) print "a is defined"
if (!exists("b")) print "b is not defined"

Valid names are the same as in most programming languages: they must begin with a letter, but subsequent characters may be letters, digits, or "_".

Each function definition is made available as a special string-valued variable with the prefix 'GPFUN_'.

Example:

set label GPFUN_sinc at graph .05,.95

See show functions, functions, gnuplot-defined variables, macros, value.