Arguments specified after a replot command will be added onto the last plot or splot command (with an implied ',' separator) before it is repeated. replot accepts the same arguments as the plot and splot commands except that ranges cannot be specified. Thus you can use replot to plot a function against the second axes if the previous command was plot but not if it was splot.
plot '-' ; ... ; replot
is not recommended, because it will require that you type in the data all over again. In most cases you can use the refresh command instead, which will redraw the plot using the data previously read in.
Note that in multiplot mode, replot can only reproduce the most recent component plot, not the full set.