Graphics with Gnuplot from C and Python language

Posted by ionut
September 30, 2008

If you are doing a lot of calculations/simulations, like me, using C or/and Python language, you also need to visualize your simulated data. One way to do it is just to generate/write a “data” file and plot it with your plotting preferred package after the simulation is finished. But maybe, sometimes, you want to see the graph in real time, so, here I would like to show a small C and Python program which calls Gnuplot and save the plot in png format.

C Example:

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#include<stdio.h>
/*
	Author: Ioan Vancea (www.vioan.ro)
	*****************************/
int main()
{
	FILE *pipe = popen("gnuplot -persist","w");
	fprintf(pipe, "set samples 40\n");
	fprintf(pipe, "set isosamples 40\n");
	fprintf(pipe, "set hidden3d\n");
	fprintf(pipe, "set xrange [-8.000:8.000]\n");
	fprintf(pipe, "set yrange [-8.000:8.000]\n");
	fprintf(pipe, "set zrange [-2.000:2.000]\n");
	fprintf(pipe, "set terminal png\n");
	fprintf(pipe, "set output 'graph.png'\n");
	fprintf(pipe, "set title 'We are plotting from C'\n");
	fprintf(pipe, "set xlabel 'Label X'\n");
	fprintf(pipe, "set ylabel 'Label Y'\n");
	/* fprintf(pipe, "plot 'datafile.dat' using 1:2\n");*/
	fprintf(pipe, "splot cos(x)+cos(y)\n");
 
  close(pipe);
  return 0;
}

The output:

Python Example (I use Gnuplot.py):

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#!/usr/bin/env python
 
'''
	Author: Ioan Vancea (www.vioan.ro)
'''
 
import math
import Gnuplot
gp = Gnuplot.Gnuplot(persist=1)
x = range(1000)
y = [math.sin(a) + math.cos(a) + math.tan(a) for a in x]
data = Gnuplot.Data(x, y, title='Plotting from Python')
gp('set data style lines')
gp('set xlabel "Label x"')
gp('set ylabel "Label y"')
gp.plot(data);
gp.hardcopy(filename="pyplot.png",terminal="png")

The output:

Gnuplot, Python, SciComputing

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Comments
Pingback by Calling Gnuplot from Perl | Ionut's WebSpace on September 30, 2008 @ 11:18 pm

[...] my previous post I discussed how to call Gnuplot from C and Python language. If you are using Perl, then, here is [...]

Comment by Nicolas on October 2, 2008 @ 11:27 am

You may want to check out this page, it has some information about calling gnuplot from C, C++, Python and others:
http://ndevilla.free.fr/gnuplot/
Hope it helps

Comment by ionut on October 2, 2008 @ 9:54 pm

Thanks Nicolas, nice, I didn’t know about “gnuplot_i”.

Comment by Craig Finch on January 7, 2009 @ 11:59 pm

I like the Pylab/Matplotlib package:
http://matplotlib.sourceforge.net/
If you are familiar with plotting in Matlab, it will be easy to pick up. I have found it to be very good for publication-quality 2D graphics (it doesn’t do 3D).

Comment by Dan on March 19, 2009 @ 11:02 pm

Hi,

I would be interested in bidirectional piping of gnuplot. I want to use gnuplot for both plotting and fitting in my code, and I want to read gnuplot’s output. Is it possible to do?
Thanks,
Dan

Comment by ionut on March 26, 2009 @ 10:42 pm

I didn’t try that, but I think you can also use some intermediate/temporary files for that.

Comment by shreyas on September 3, 2009 @ 5:06 am

Hey i wanted to know if there is a way of plotting graphs from my c++ program without saving the data on to a file.. I am generating random numbers and calculating their averages… I need the averages to be plotted on the graph but i dont want to save the averages to a file.

Thanks

Comment by Trygve on March 17, 2011 @ 12:00 am

I tried ypur code in an Xcode project I’m working on.

I get the error:

sh: gnuplot: command not found

Can anyone help me!

Thanks

Comment by ionut on March 17, 2011 @ 12:05 am

It means that you don’t have gnuplot installed or maybe is not in your PATH. Try to install it using macports or fink. Check their website and the documentation to see how to use them.

Comment by Philip on March 22, 2011 @ 2:53 am

Thanks, just what I was looking for. I found gnuplot_i first but this is way simpler :)
By the way if you want to plot data from your c-program, first tell gnuplot to read it from stdin:
fprintf(pipe, “plot ‘-’\n”);
Then print all the data you want (probably in a loop)
fprintf(pipe, “%lf %lf\n”, variable1, variable2);
…
When you’re done tell gnuplot that data ends here
fprintf(pipe, “e\n”);

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