Gnuplot For Visualizing Data

When writing research papers and technical documents, displaying data in a useful manner is really important. Specially when presenting different evaluation results in a statistical form in documents we  need to draw different types of graphs. Since it is important to me, I was looking for a good tool for that for sometime. Since I'm using Ubuntu Linux, the best recommended one I could find on the Internet was Gnuplot. It's a free tool and very powerful to draw different types of graphs.

Since it is a command line tool it takes some time to learn about how to use Gnuplot for our requirements. Therefore I postponed my Gnuplot learning process several times and finally today I got some first hand experience. At first I found this blog post. Then it sent me to this tutorial which contained so many helpful information. I recommend it if you really want to learn to use Gnuplot tool from the beginning.

To make my mind easy to memorise what I have learnt up to now, I decided to write a little summary of the basic things I need to use Gnuplot. So, here we go.

 1. To install gnuplot on my Ubuntu-11.04 system, I issued the following command in the terminal.

         sudo apt-get install gnuplot-x11

2. No I can start gnuplot by issuing the following command.

    gnuplot

It will show a prompt like the one showed in the above picture and all the other thing has to be done on this prompt. If you want to exit from the gnuplot, type 'exit' on the prompt.

3. Now let's create an example plot and write it in to an image file. To do that issue the following commands on the prompt.

    set terminal jpeg 

    set output 'our_file_name.jpg'

    plot sin(x)

Now in a new terminal you can go to the working directory of the gnuplot and open the 'our_file_name.jpg' file to view your graph. You can find out the working directory of gnuplot by typing 'pwd' on the prompt just like you do in normal terminal. To change the working directory use 'cd' as follows on the prompt.

cd "/path/to/new/directory"

4. To add the names of the graph, the x axis and the y axis use the following commands.

         set title "My graph title"

    set xlabel 'x axis'

    set ylabel 'y axis'

After changing these settings we have to re-run the previous plot command with new changes. We can do it in shorten way by using 'replot' command as below so that you don't have to write a plot command again and again since sometimes plot command is too long with so many parameters.

    set terminal jpeg

    set output 'plot.jpg'

    replot

5. Even though we set different parameters in gnuplot as above, they are gone if we exit from gnuplot and start it again later. Therefore it's useful to save current configurations so that we can load them again.

View the current configuration by issuing the following command.

    show all

You can save the configuration to a file as below.

     save "savefile.plt"

You can load a saved configuration from a file by the following command

      load "savefile.plt"

6. Now we want to draw a graph using the data which comes from another data file. Consider the following file named 'my_data.dat'.

 1 10 11   
 2 20 23   
 3 30 34   
 4 40 45   
 5 50 56   
 6 60 67        
 7 70 78   
 8 80 89   
 9 90 100  

We have three columns of data in that file. Let's draw a graph with first two columns.

      set terminal jpeg
     set output 'first_graph.jpg'
     plot "./my_data.dat" using 1:2 title "First line" with lines

first_graph.jpg

Now let's draw two graphs in the same figure. The first graph use first column as x-axis and second column as y-axis while the second graph use the first column as x-axis and third column as y-axis. Here's how we do that. 

     set terminal jpeg
     set output 'second_graph.jpg'
     plot "./my_data.dat" using 1:2 title "First line" with lines, "./my_data.dat" using 1:3 title "Second line" with lines

second_graph.jpg

There are so many other things to learn in Gnuplot to create nice graphs. I believe that learning gnupot would not be a wastage of time since good data visualisation is an important skill which should be in a computer scientist. So, I should expertise it in the future.  :)

That's all for now. Enjoy drawing graphs with gnuplot!