Boxplot

Boxplots are a common way to represent a statistical distribution of values. Quartile boundaries are determined such that 1/4 of the points have a value equal or less than the first quartile boundary, 1/2 of the points have a value equal or less than the second quartile (median) value, etc. A box is drawn around the region between the first and third quartiles, with a horizontal line at the median value. Whiskers extend from the box to user-specified limits. Points that lie outside these limits are drawn individually.

Examples

# Place a boxplot at x coordinate 1.0 representing the y values in column 5
plot 'data' using (1.0):5
# Same plot but suppress outliers and force the width of the boxplot to 0.3
set style boxplot nooutliers
plot 'data' using (1.0):5:(0.3)

By default only one boxplot is produced that represents all y values from the second column of the using specification. However, an additional (fourth) column can be added to the specification. If present, the values of that column will be interpreted as the discrete levels of a factor variable. As many boxplots will be drawn as there are levels in the factor variable. The separation between these boxplots is 1.0 by default, but it can be changed by set style boxplot separation. By default, the value of the factor variable is shown as a tic label below (or above) each boxplot.

Example

# Suppose that column 2 of 'data' contains either "control" or "treatment"
# The following example produces two boxplots, one for each level of the
# factor
plot 'data' using (1.0):5:(0):2

The default width of the box can be set via set boxwidth <width> or may be specified as an optional 3rd column in the using clause of the plot command. The first and third columns (x coordinate and width) are normally provided as constants rather than as data columns.

By default the whiskers extend from the ends of the box to the most distant point whose y value lies within 1.5 times the interquartile range. By default outliers are drawn as circles (point type 7). The width of the bars at the end of the whiskers may be controlled using set bars.

These default properties may be changed using the set style boxplot command. See set style boxplot, bars, boxwidth, fillstyle, candlesticks.