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Gnuplot.jl/docs/src/basic.md
Giorgio Calderone 1e7a8a481f Docstrings updated
2020-03-26 23:09:04 +01:00

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Basic usage

The main purpose of the Gnuplot.jl package is to send data and commands to the underlying gnuplot process, in order to generate plots. Unlike other packages, however, the actual commands to plot, or the plot attributes, are not specified through function calls. This is what makes Gnuplot.jl easy to learn and use: there are no functions or keywords names to memorize1.

The most important symbols exported by the package are the @gp (for 2D plots) and @gsp (for 3D plots) macros, both accepting any number of arguments, and whose meaning is interpreted as follows:

  • one, or a group of consecutive, array(s) build up a dataset. The different arrays are accessible as columns 1, 2, etc. from the gnuplot process. The number of required input arrays depends on the chosen plot style (see gnuplot documentation);

  • a string occurring before a dataset is interpreted as a gnuplot command (e.g. set grid);

  • a string occurring immediately after a dataset is interpreted as a plot element for the dataset, by which you can specify using clause, with clause, line styles, etc.;

  • the special symbol :-, whose meaning is to avoid starting a new plot (if given as first argument), or to avoid immediately running all commands to create the final plot (if given as last argument). Its purpose is to allow splitting one long statement into multiple (shorter) ones.

The above lists all the required concepts to follow the examples presented below. The @gp and @gsp macros also accepts further arguments, but their use will be discussed in Advanced techniques.

[2D plots](@id plots2d)

Here we will show a few examples to generate 2D plots. The examples are intentionally very simple to highlight the behavior of Gnuplot.jl. See Examples for more complex ones.

Remember to run:

using Gnuplot

before running the examples.

using Gnuplot
Gnuplot.quitall()
mkdir("assets")
Gnuplot.splash("assets/logo.png")
saveas(file) = save(term="pngcairo size 480,360", output="assets/$(file).png")
empty!(Gnuplot.options.init)
Gnuplot.exec("set term unknown")

Simple examples involving just gnuplot commands:

Plot a sinusoid:

@gp "plot sin(x)"
saveas("basic1") # hide

Plot two curves:

@gp "set key left" "plot sin(x)" "pl cos(x)"
saveas("basic2") # hide

!!! note Note that all gnuplot commands can be abbreviated as long as the resulting string is not ambiguous. In the example above we used pl in place of plot.

Split a @gp call in three statements:

@gp    "set grid"  :-
@gp :- "p sin(x)"  :-
@gp :- "plo cos(x)"
saveas("basic3") # hide

Send data from Julia to gnuplot:

Plot a parabola

@gp (1:20).^2
saveas("basic4") # hide

Plot a parabola with scaled x axis, lines and legend

x = 1:20
@gp "set key left"   x ./ 20   x.^2   "with lines tit 'Parabola'"
saveas("basic5") # hide

Multiple datasets, logarithmic axis, labels and colors, etc.

x = 1:0.1:10
@gp    "set grid" "set key left" "set logscale y"
@gp :- "set title 'Plot title'" "set label 'X label'" "set xrange [0:12]"
@gp :- x x.^0.5 "w l tit 'Pow 0.5' dt 2 lw 2 lc rgb 'red'"
@gp :- x x      "w l tit 'Pow 1'   dt 1 lw 3 lc rgb 'blue'"
@gp :- x x.^2   "w l tit 'Pow 2'   dt 3 lw 2 lc rgb 'purple'"
saveas("basic6") # hide

!!! note The above example lacks the trailing :- symbol. This means the plot will be updated at each command, adding one curve at a time.

Keywords for common commands

In order to avoid typing long, and very frequently used gnuplot commands, Gnuplot.jl provides a few keywords which can be used in both @gp and @sgp calls:

  • xrange=[low, high] => "set xrange [low:high];
  • yrange=[low, high] => "set yrange [low:high];
  • zrange=[low, high] => "set zrange [low:high];
  • cbrange=[low, high]=> "set cbrange[low:high];
  • key="..." => "set key ...";
  • title="..." => "set title \"...\"";
  • xlabel="..." => "set xlabel \"...\"";
  • ylabel="..." => "set ylabel \"...\"";
  • zlabel="..." => "set zlabel \"...\"";
  • xlog=true => set logscale x;
  • ylog=true => set logscale y;
  • zlog=true => set logscale z;

All such keywords can be abbreviated to unambiguous names.

By using the above keywords the first lines of the previous example:

@gp    "set grid" "set key left" "set logscale y"
@gp :- "set title 'Plot title'" "set label 'X label'" "set xrange [0:12]"

can be replaced with a shorter version:

@gp    "set grid" k="left" ylog=true
@gp :- tit="Plot title" xlab="X label" xr=[0,12]

Plot images

Gnuplot.jl can also display images, i.e. 2D arrays:

img = randn(Float64, 30, 50)
img[10,:] .= -4
@gp img "w image notit"
saveas("basic7a") # hide

Note that the first index corresponds to the X coordinate when the image is displayed.

The following example shows how to fix orientation of an image by means of the using clause (the TestImages package is required to run this example):

using TestImages
img = testimage("lena");
@gp "set size square" "set autoscale fix" img "u 2:(-\$1):3:4:5 with rgbimage notit"
save(term="jpeg size 480,360", output="assets/basic7b.jpg") # hide

[3D plots](@id plots3d)

3D plots follow the same rules as 2D ones, just replace the @gp macro with @gsp and add the required columns (according to the plotting style).

E.g., to plot a spiral increasing in size along the X direction:

x = 0:0.1:10pi
@gsp x  sin.(x) .* x  cos.(x) .* x  x./20  "w p pt 7 ps var lc pal"
saveas("basic8") # hide

The keywords discussed above can also be used in 3D plots.

Palettes and line types

The Gnuplot.jl package comes with all the ColorSchemes palettes readily available.

A gnuplot-compliant palette can be retrieved with palette(), and used as any other command. The previous example may use an alternative palette with:

x = 0:0.1:10pi
@gsp palette(:viridis) x  sin.(x) .* x  cos.(x) .* x  x./20  "w p pt 7 ps var lc pal"
saveas("basic8a") # hide

The ColorSchemes palettes can also be used to generate line types (actually just color attributes), by means of the linetypes() function, e.g.

@gp linetypes(:deepsea)
x = 1:0.1:4pi
for i in 1:5
    @gp :- x i.* sin.(x) "w l notit lw 5"
end
saveas("basic9") # hide

Exporting plots to files

The save() function allows to export all plots (as well as multiplots, see Multiplot) to a file using one of the many available gnuplot terminals. To check which terminals are available in your platform type set term in your gnuplot terminal.

All plots in this page have been saved with:

save(term="pngcairo size 480,360", output="assets/output.png")

except the Lena image, saved with the jpeg terminal:

save(term="jpeg size 480,360", output="assets/output.png")

Gnuplot scripts

Besides exporting plots in a file Gnuplot.jl can also save a script, i.e. a file containing the minimum set of data and commands required to generate a plot within gnuplot.

To generate a script for one of the example above use:

save("script.gp")

after the plot has been displayed. The script can then be used within a gnuplot session as follows:

gunplot> load 'script.gp'

to generate a plot identical to the original one, without using the Julia language.

The purpose of gnuplot scripts is to allow sharing all data, alongside a plot, in order to foster collaboration among scientists and replicability of results. Moreover, a script can be used at any time to change the details of a plot, without the need to re-run the Julia code used to generate it the first time.

Finally, the scripts are the only possible output when Dry sessions are used (i.e. when gnuplot is not available in the user platform.

  1. a previous knowledge of gnuplot usage is, nevertheless, required. ↩︎