\[
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%
%%%% This text file is part of the source of
%%%% `Introduction to High-Performance Scientific Computing'
%%%% by Victor Eijkhout, copyright 2012-2020
%%%%
%%%% mathjax.tex : macros to facility mathjax use in html version
%%%%
%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newcommand\inv{^{-1}}\newcommand\invt{^{-t}}
\newcommand\bbP{\mathbb{P}}
\newcommand\bbR{\mathbb{R}}
\newcommand\defined{
\mathrel{\lower 5pt \hbox{${\equiv\atop\mathrm{\scriptstyle D}}$}}}
\newcommand\macro[1]{$\langle$#1$\rangle$}
\newcommand\dtdxx{\frac{\alpha\Delta t}{\Delta x^2}}
\]
27.1 : Usage modes

27.2 : Plotting

27.2.1 : Plotting curves

27.2.2 : Plotting data points

27.2.3 : Customization

27.3 : Workflow

Back to Table of Contents# 27 Plotting with GNUplot

## 27.1 Usage modes

## 27.2 Plotting

### 27.2.1 Plotting curves

### 27.2.2 Plotting data points

### 27.2.3 Customization

## 27.3 Workflow

Back to Table of Contents
27.2 : Plotting

27.2.1 : Plotting curves

27.2.2 : Plotting data points

27.2.3 : Customization

27.3 : Workflow

Back to Table of Contents

The
*gnuplot*
utility is a
simple program for plotting sets of points or curves. This very short
tutorial will show you some of the basics. For more commands and
options, see the manual
http://www.gnuplot.info/docs/gnuplot.html
.

crumb trail: > gnuplot > Usage modes

The two modes for running
`gnuplot`
are
*interactive*
and
*from file*
command line, type commands, and watch output appear; you terminate an
interactive session with
`quit`
. If you want to save the results of
an interactive session, do
`save "name.plt"`
. This file can be
edited, and loaded with
`load "name.plt"`
.

Plotting non-interactively, you call
`gnuplot `
.

The output of
`gnuplot`
can be a picture on your screen, or drawing
instructions in a file. Where the output goes depends on the setting
of the
*terminal*
. By default,
`gnuplot`
will try to draw a
picture. This is equivalent to declaring

set terminal x11

or
`aqua`
,
`windows`
, or any choice of graphics hardware.

For output to file, declare

set terminal pdf

or
`fig`
,
`latex`
,
`pbm`
, et cetera. Note that this will only
cause the pdf commands to be written to your screen: you need to
direct them to file with

set output "myplot.pdf"

or capture them with

gnuplot my.plt > myplot.pdf

crumb trail: > gnuplot > Plotting

The basic plot commands are
`plot`
for 2D, and
`splot`
(`surface
plot') for 3D plotting.

crumb trail: > gnuplot > Plotting > Plotting curves

By specifying

plot x**2

you get a plot of $f(x)=x^2$;
`gnuplot`
will decide on the range
for $x$.
With

set xrange [0:1] plot 1-x title "down", x**2 title "up"

you get two graphs in one plot, with the $x$ range limited to $[0,1]$,
and the appropriate legends for the graphs. The variable
`x`
is the
default for plotting functions.

Plotting one function against another -- or equivalently, plotting a parametric curve -- goes like this:

set parametric plot [t=0:1.57] cos(t),sin(t)

which gives a quarter circle.

To get more than one graph in a plot, use the command
`set multiplot`
.

crumb trail: > gnuplot > Plotting > Plotting data points

It is also possible to plot curves based on data points. The basic
syntax is
`plot 'datafile'`
, which takes two columns from the data
file and interprets them as $(x,y)$ coordinates. Since data files can often have
multiple columns of data, the common syntax is \n{plot 'datafile'
using 3:6} for columns 3 and 6. Further qualifiers like \n{with
lines} indicate how points are to be connected.

Similarly,
`splot "datafile3d.dat" 2:5:7`
will interpret three
columns as specifying $(x,y,z)$ coordinates for a 3D plot.

If a data file is to be interpreted as level or height values on a
rectangular grid, do
`splot "matrix.dat" matrix`
for data points;
connect them with

split "matrix.dat" matrix with lines

crumb trail: > gnuplot > Plotting > Customization

Plots can be customized in many ways. Some of these customizations use
the
`set`
command. For instance,

set xlabel "time" set ylabel "output" set title "Power curve"

You can also change the default drawing style with

set style function dots

(
`dots`
,
`lines`
,
`dots`
,
`points`
, et cetera), or
change on a single plot with

plot f(x) with points

crumb trail: > gnuplot > Workflow

Imagine that your code produces a dataset that you want to plot, and you run your code for a number of inputs. It would be nice if the plotting can be automated. Gnuplot itself does not have the facilities for this, but with a little help from shell programming this is not hard to do.

Suppose you have data files

data1.dat data2.dat data3.dat

and you want to plot them with the same gnuplot commands. You could
make a file
`plot.template`
:

set term pdf set output "FILENAME.pdf" plot "FILENAME.dat"

The string
`FILENAME`
can be replaced by the actual file names
using, for instance
`sed`
:

for d in data1 data2 data3 ; do cat plot.template | sed s/FILENAME/$d/ > plot.cmd gnuplot plot.cmd done

Variations on this basic idea are many.