3.5 How to make Figures with Gnuplot 91

set terminal pslatex

set output "derivative.tex"

set xrange [-15:0]

set yrange [-10:8]

set xlabel "log$_{10}(h)$"

set ylabel "$\epsilon$"

plot "out.dat" title "Relative error" w l

generates aLATEXfilederivative.tex. Alternatively, you could write the above commands in
a filederivative.gnuand useGnuplotas follows

gnuplot>load ’derivative.gnu’

You can then include this file in aLATEXdocument as shown here

\begin{figure}

\begin{center}

\input{derivative}

\end{center}

\caption{Log-log plot of the relative error of the second

derivative of $e^x$ as function of decreasing step

lengths $h$. The second derivative was computed for

$x=10$ in the program discussed above. See text for

further details\label{fig:lossofprecision}}

\end{figure}

Most figures included in this text have been generated using gnuplot.
Many of the above commands can all be baked in a Python code. The following example
reads a file from screen withxandydata, and plots these data and saves the result as a
postscript figure.

#!/usr/bin/env python

importsys
fromNumericimport*
importGnuplot

g = Gnuplot.Gnuplot(persist=1)

try:
infilename = sys.argv[1]
except:
print"Usage of this script", sys.argv[0],"infile", sys.argv[1]; sys.exit(1)

Read file with data

ifile = open(infilename,'r')

Fill in x and y

x = [] ; y = []
forlineinifile:
pair = line.split()
x = float(pair[0]); y = float(pair[1])
ifile.close()

convert to a form that the gnuplot interface can deal with

d = Gnuplot.Data(x, y, title='data from output file', with='lp')
g.xlabel('log10(h)')# make x label
g.ylabel('log10(|Exact-Computed|)/|Exact|')
g.plot(d) # plot the data
g.hardcopy(filename="relerror.ps",terminal="postscript", enhanced=1, color=1)