gnuplot is a great graphing utility that I have been using for a long time now. In fact, many figures of my diploma thesis were created using this tool. One of the drawbacks of gnuplot, however, is the lack of colour palettes that are suitable for scientific publishing. As Kenneth Moreland points out in his well-founded and entertaining paper about diverging colour maps for scientific visualization, there are better choices than the default rainbow colour map.

Consequently, I tried adding a new colour palette to gnuplot, starting with Moreland's cool-to-warm colour map. I simply used the RGB values provided in the paper and normalized them. Then, I issued the following gnuplot command:

set palette defined(\
0       0.2314  0.2980  0.7529,\
0.03125 0.2667  0.3529  0.8000,\
0.0625  0.3020  0.4078  0.8431,\
0.09375 0.3412  0.4588  0.8824,\
0.125   0.3843  0.5098  0.9176,\
0.15625 0.4235  0.5569  0.9451,\
0.1875  0.4667  0.6039  0.9686,\
0.21875 0.5098  0.6471  0.9843,\
0.25    0.5529  0.6902  0.9961,\
0.28125 0.5961  0.7255  1.0000,\
0.3125  0.6392  0.7608  1.0000,\
0.34375 0.6824  0.7882  0.9922,\
0.375   0.7216  0.8157  0.9765,\
0.40625 0.7608  0.8353  0.9569,\
0.4375  0.8000  0.8510  0.9333,\
0.46875 0.8353  0.8588  0.9020,\
0.5     0.8667  0.8667  0.8667,\
0.53125 0.8980  0.8471  0.8196,\
0.5625  0.9255  0.8275  0.7725,\
0.59375 0.9451  0.8000  0.7255,\
0.625   0.9608  0.7686  0.6784,\
0.65625 0.9686  0.7333  0.6275,\
0.6875  0.9686  0.6941  0.5804,\
0.71875 0.9686  0.6510  0.5294,\
0.75    0.9569  0.6039  0.4824,\
0.78125 0.9451  0.5529  0.4353,\
0.8125  0.9255  0.4980  0.3882,\
0.84375 0.8980  0.4392  0.3451,\
0.875   0.8706  0.3765  0.3020,\
0.90625 0.8353  0.3137  0.2588,\
0.9375  0.7961  0.2431  0.2196,\
0.96875 0.7529  0.1569  0.1843,\
1       0.7059  0.0157  0.1490\
) 

I used values from [0,1] in the first column because I was normalizing my data file to this interval. More precisely, the data file I was going to plot now contained 3 columns: Columns 1 and 2 contained the coordinates of the points I was going to plot, while column 3 contained a value from [0,1]. To force gnuplot to use this third column for choosing a colour from the palette defined above, I used the following command:

plot "data.data" using 1:2:3 with points palette

That's it. Unfortunately, there does not seem to be a way to permanently store these palettes. For now, I am simply including them in my data files. At least this has the benefit of making the plots reproduceable.