Plotting

We can use gnuplot to make simple plots.

Important

By default, gnuplot will display the plot in a window. This uses the Unix X Window System. This can work when we run directly on the machines in the MathLab.

If you use portal or portal2 then, when you ssh into these remote machines, you need to add -Y to enable forwarding of windows, e.g.,

ssh -Y username@portal.mathlab.stonybrook.edu

If you are using your own machine, you will need to install a X server. See our Using the remote servers documentation.

If you are unable to get this to work, you can still have gnuplot make an image file directly (e.g. PNG) instead, as shown below.

Interactive plotting

Typically, we will plot columns from our output, making one column the x-axis and the other column the y-axis.

Let’s rerun our code, but redirect the output to a file:

./truncation_vs_roundoff > diff.out

If we look at the file diff.out (you can use cat, e.g.), then we see there are 2 columns.

We can start gnuplot as:

gnuplot

The gnuplot program will give us a commandline interface that lets us make plots, and now the prompt will be gnuplot>. To make our plot, we can do:

plot 'diff.out' using 1:2 w l

There are a few parts to this:

• This will use the data in diff.out

• The using 1:2 part means that the first column (gnuplot uses 1-based indexing) will be on the x-axis and the second column will be on the y-axis.

• The w l part means “with lines”, so the data will be draw using line segments instead of plotting points.

This plot will not look like what we saw previously, since it is using a linear scale, and we really want a log-scale to see the range of data. We can switch by doing:

set logscale
replot

By default, gnuplot shows a legend based on the plot command. We can make this more descriptive by adding the title keyword:

plot 'diff.out' using 1:2 title "error" w l

or remove the title as:

plot 'diff.out' using 1:2 notitle w l

Now we can add axis labels:

set xlabel "dx"
set ylabel "error"
replot

Saving the plot

To output as an image file (PNG), we can do:

set term png enhanced
set output "diff.png"
replot

Then you can exit gnuplot either by typing exit or using Ctrl-d.

You should then see the file diff.png in your directory, which should look like the plot from the previous page:

Using a plotting script

We can put all of the plotting commands into a file, like diff.gp (the extension, .gp is not standard, but here we use it to indicate gnuplot). An example script is:

Listing 67 diff.gp

set term png enhanced size 800,600
set output "diff.png"

set logscale
set grid

set xlabel "dx"
set ylabel "error"

plot "diff.out" using 1:2 notitle with lines

and you would use this by doing:

gnuplot diff.gp

The key part of this is that the plot command comes at the very end.

Tip

If you could not get the X Window forwarding to work, then adapting the script above it the easiest approach. You can then scp the plot (diff.png in this case) to your local machine.