Gnuplot Lab


gnuplot is a tool that allows us to very quickly visualise data, it can be very useful for testing program output without having to generate complex graphical interface code, the following labs is going to use gnuplot to help visualise the output of several sample programs.

Getting started

For these demos we will be using text files to store 2D point positions and will load them into gnuplot to draw, to get started create the following file

touch line.dat

Open line.dat in an editor and add the following code

# is used for comments
# this indicates the format of the data
# X Y
-2.0 1.0
3.0 -2.0

Next we need to add an alias into our .bashrc so we can use gnuplot

alias gnuplot='/public/bin/gnuplot/bin/gnuplot'

Save the .bashrc and source ~/.bashrc In the shell type gnuplot and you should see the gnuplot > prompt, at the prompt enter the following code to plot our data file

gnuplot> plot 'line.dat' with lines

Which should give you the following image


Random points

The following program will generate a series of random points using the new C++ 11 random features this should now always be used instead of rand as rand should now be considered harmful!

/// to compile use clang++ -Wall -g -std=c++11  random.cpp -o rand
#include <iostream>
#include <cstdlib>
#include <random>

int main()
  std::random_device rd;
  std::default_random_engine randEngine(rd()) ;
  std::uniform_real_distribution<> dist(-10, 10);

  std::cout<<"# X Y \n";
  for(int i=0; i<200; ++i)
    std::cout<<dist(randEngine)<<' '<<dist(randEngine)<<'\n';
  return EXIT_SUCCESS;

Once the program is compiled execute and pipe the output to a file using the following command line

./rand &>rand.dat

The file can now be plotted in gnuplot using the following

gnuplot> plot 'rand.dat' 

And will give the following result




The parametric equation for a circle can be generated using the following algorithm

get radius as double
set t = 0.01
for t=0.0 -> 2 * M_PI 
  output x y

Write a C++ program to output to the console the values for x and y into circle.dat using the following C++ commands

Multiple Plots

It is possible to plot more than one column of data in a single plot by using the following format

0 0 1
0.01 0.0627905 0.998027
0.02 0.125333 0.992115

Then with gnuplot use the command

gnuplot> plot 'sine.dat' using 1:2 with lines , 'sine.dat' using 1:3 with lines

Write a program that requests two inputs from the user one for frequency and one for amplitude, and then plots both the cos and sine waves for the given values using the algorithm

for x=0 x<range x+=0.1

Writing to files

Using the examples above, modify the programs to write the data directly to a file, for more information on C++ file I/O routines use the following resources


  • Modify the random example above to generate 3D points, these may be plotted using the splot function
  • investigate other algorithms for generating 3D points example and try to plot them

Solutions / Samples

Solutions and samples can be found here, please try to do it yourself first.