gnuplot quickstart guide

Sometimes it is really nice to just take a quick look at some data. However, when working on remote computers, it is a bit of a burden to move data files to a local computer to create a plot in something like R. One solution is to use gnuplot and make a quick plot that is rendered in the terminal. It isn’t very pretty by default, but it gets the job done quickly and easily. There are also advanced gnuplot capabilities that aren’t covered here at all.

gnuplot has it’s own internal syntax that can be fed in as a script, which I won’t get into. Here is the very simplified gnuplot code we’ll be using:

set terminal dumb size 120, 30; set autoscale; plot '-' using 1:3 with lines notitle

Let’s break this down:

• set terminal dumb size 120, 30: gnuplot has ‘terminals’, which is essentially the output format for the plot. Here we are using dumb which renders the plot in ASCII characters in the terminal. You can also specify size parameters for the plot, in this case we’re using size 120, 30 to make it a bit larger than default (you can play around with this).
• set autoscale: This just makes it so that the axes are automatically scaled, which is normally desireable.
• plot '-' using 1:3 with lines notitle: Performs the plotting magic. The '-' is the file from which to take the data and plot, which is being read in from STDIN ('-'), but you could specify a file name instead. Next, using 1:3 tells gnuplot to use columns 1 and 3 from the data file for plotting (x = 1, y = 3). Change accordingly to plot any column combination you desire. Finally, with lines notitle just makes the plot a line plot with no title.

This should allow for many basic plots (but note the lack of axis labels!). This plotting script is called as follows:

gnuplot -p -e "set terminal dumb size 120, 30; set autoscale; plot '-' using 1:3 with lines notitle"

This basically feeds the script from above into the gnuplot command-line call. The -p flag just allows the plot to persist beyond the command call (otherwise it disappears) and -e tells gnuplot to expect the following script, which is surrounded by quotes.

This makes it pretty easy to call plotting in a loop, as follows:

for i in *.summary.txt; do echo $i; \
  cat $i | gnuplot -p -e "set terminal dumb size 120, 30; set autoscale; plot '-' using 1:3 with lines notitle"; done

Here, we are taking each file that ends in .summary.txt, and piping it’s content into gnuplot to produce a plot. Pretty cool, right!?