gnuplot Cookbook: Visual guide to every kind of graph you can make with this plotting software with this book and ebook

The graphs we created in the previous chapter served the purpose of illustrating the various 2D plot styles, but they were all missing a few things. For a graph to be useful in the real world, it must be endowed with a title and various labels that explain what it is about and how to interpret its assorted plot elements.

In this chapter, we revisit the 2D graph, creating one with a few curves, two different y-axes, and shaded areas, and in each recipe, we add an additional informative decoration to the plot. The graph remains the same, but in each recipe it will acquire more detailed annotation. This chapter is all about how to make these annotations, not about the plotting itself.

In this recipe, we add labels to our axes to explain what is being plotted and the significance of the tics and numerical scales. We also add an overall title that will appear at the top of the graph.

set yrange [-1.5:1.5]
set xrange [0:6.3]
set ytics nomirror
set y2tics 0,.1
set y2range [0:1.2]
set style fill pattern 5
set xlabel "Time (sec.)"
set ylabel "Amplitude"
set y2label "Error Magnitude"
set title "Fourier Approximation to Square Wave"
plot 'ch2.dat' using 1:2:(sgn($2)) with filledcurves,\'' using 1:2 with lines...

The default font size for labels and titles in gnuplot looks a little small with most terminals. For example, the labels can be hard to read from the back rows of an auditorium during a presentation. We now show how to adjust the font size, and how to select the font for labels and titles.

set term postscript landscape "Courier, 18"
set output 'squarewave.ps'
set yrange [-1.5:1.5]
set xrange [0:6.3]
set ytics nomirror
set y2tics 0,.1
set...

The legend refers to the block of information printed on the graph (or occasionally outside it) that explains which curve or symbol is associated with which quantity. It is called a key in gnuplot. A legend or some device that conveys the equivalent information is essential when the graph displays more than one curve.

You've probably noticed that all of our example graphs already contain a key; this is done by gnuplot by default. This recipe will show you how to take complete control of your graph's legend.

set term postscript landscape
set yrange [-1.5:1.5]
set xrange [0:6.3]
set ytics nomirror
set y2tics 0,.1
set y2range [0:1.2]
set style fill pattern 5
set key at graph .9, .9 spacing 3 font "Helvetica, 14"
set xlabel "Time (sec.) font "Courier, 12"
set ylabel "Amplitude" font "Courier, 12"
set y2label "Error Magnitude" font "Courier, 12"
set title "Fourier Approximation...

This recipe is a simple modification of the previous one, but gets its own section because the legend in the box is such a popular style (for better or worse).

set key spacing 3 font "Helvetica, 14"
set key box lt -1 lw 2

Complex graphs can often benefit from information in addition to what can be provided in a title, in the axis labels, and a legend. Sometimes we need to explain the meaning of a particular feature on the graph. This can be done with labels and arrows.

set term postscript landscape
set out 'fourier.ps'
set yrange [-1.5:1.5]
set xrange [0:6.3]
set ytics nomirror
set y2tics 0,.1
set y2range [0:1.2]
set style fill pattern 5
set key at graph .9, .9 spacing 3 font "Helvetica, 14"
set xlabel "Time (sec.)" font "Courier, 12"
set ylabel "Amplitude" font "Courier, 12"
set y2label "Error Magnitude" font "Courier, 12"
set title "Fourier Approximation to Square Wave" font "Times-Roman, 24"
set label "Approximation error" right at 2.4, 0.45 offset -.5, 0
set arrow 1 from first 2.4, 0.45 to 3, 0.93 lt 1 lw 2 front size .3, 15
plot 'ch2.dat' using 1:2:(sgn($2)) with filledcurves notitle...

A new feature in gnuplot 4.4 allows you to use the Unicode character set in your graph's title and labels. This is a vast improvement over more cumbersome methods of entering special characters, but it does not work on all terminals. For example, PostScript does not support Unicode directly, but some implementations of the pdf and png terminals, and others, will work. The following example was created using the pngcairo version of the png terminal. Which versions you have available will depend on the details of your operating system and gnuplot installation. A more general method for creating complex labels is given in the next recipe.

Certain terminals support enhanced text, which means that they accept a markup language specific to gnuplot that can be used to include characters from various fonts, set subscripts, superscripts, overprint characters, and generally manipulate text with sufficient flexibility to create simple equations to serve as annotations for your graphs. This recipe shows how to make such an equation, but you will see that the markup is cumbersome, and attaining the desired result requires some amount of trial and error. Frankly, a better way to create all but the simplest mathematical text is to use the LaTeX techniques covered in a later chapter, which also leads to a much more attractive output.

However, the enhanced text mode can be useful for quickly placing some mathematical text on your plot, in situations where you are not too picky about the typographical quality of the result.