Gnuplot Tricks

This page is a repository of collected tricks for graphing data in Gnuplot. An extremely useful  Gnuplot tricks page is another must see. Also, the  Gnuplot FAQ contains lots of other frequently and not-so frequently asked questions.

• Plotting a function of a datafile column: Fairly intuitive. The things to watch out for are: (i) surround your expression in brackets, (ii) use $i to refer to the i'th column. The following will plot the inverse of the sixth column against the log of the fifth.

  plot "myfile.dat" using (1/$6):(log($5))
  

• Shade certain regions of a graph: export it to SVG using

  set terminal svg size 640 480 fname 'Times New Roman' fsize 8
  

  then editing the resulting SVG file in Inkscape, and exporting to PostScript.

• Add labels to particular regions of a graph, or move the key,  see the FAQ mentioned above

• Making histograms is not possible, but a perl script that generates the relevant figures is the way forward. There are many different existing ones, talk to Jon, David, or Andy (at least!).

• Generating heat maps from 3-dimensional data is easy with Gnuplot 4.0.  See the FAQ mentioned above, which will result in something like

  unset key
  set dgrid3d
  set hidden3d
  set pm3d map
  splot "data.plot" using 1:2:3
  

  Another useful link for pm3d (including how to stop it interpolating to produce non-existant values!) is  http://gnuplot.sourceforge.net/demo_4.2/pm3d.html. Note that dgrid3d uses a norm of ALL the input values to define each point (using an inverse-distance weighting). This is unlikely to be what you want if the distribution of your data over the plotting space is uneven! Instead, try:

  set pm3d map
  splot "data.plot" using 1:2:3 with points ps 2 pt 5 palette
  

which gives coloured points (in relation to the actual value of the point). If you unset pm3d map you'll see the points (with their colours) in 3-D.

• Changing the background colour is actually not as difficult as it might seem. You can  define a rectangle of appropriate colour, and change the line and border colours to match. You can also  change the palette used by pm3d. An example of a heat map with a colour scheme suitable for monochrome printing is:

  set obj 1 rectangle behind from graph 0,0 to graph 1,1
  set obj 1 fillstyle solid 1.0 fillcolor rgb "black"
  unset key
  set pm3d map
  set border linecolor rgb "black"
  set palette rgbformulae 30,31,32
  set tics out
  set cbtics in
  splot "data.plot" using 1:2:3 with p ps 3 pt 5 palette
  

• Bar charts that are multiple bars per independent variable value can be made with a very flexible  bar chart script from Derek Bruening