Discrete heat map with GNUPLOT

Question

I'm trying to make something as a heat map with GNUPLOT but I need that my palette takes discrete colors for defined values.

I mean, my data file has three columns, for example:

x y value

0.0 0.0 10
0.0 0.5 2
0.0 1.0 2
0.5 1.0 10
1.0 0.0 -1
1.0 1.0 -1

I need that each point has one color depending of its value. Traditional heat map mixes point making regions of continuos colors, but I need it in a discrete form.

ewcz ewcz · Accepted Answer · 2017-03-12T10:12:05

If your data forms a "matrix", i.e., there are M x-samples, N y-samples, and you have the data for all MxN points, then probably the easiest solution is to use

plot ... w rgbimage u 1:2:(r($3)):(g($3)):(b($3))

and supply the r,g,b values as three additional columns as shown above.

However, if your data is "sparse" (only some of the samples are available as shown in your question) and there are not many points, one might be tempted to generate the elementary squares forming the plot manually. To this end, one could proceed as:

set terminal png enhanced
set output 'plot.png'

#custom value -> color mapping
rgb(r, g, b) = 65536 * int(r) + 256 * int(g) + int(b)
fn(val) = rgb(100 + val*10, 0, 0)

#square size
delta = 0.5
set xr [-delta/2:1+delta/2]
set yr [-delta/2:1+delta/2]

set xtics 0,delta/2,1 out nomirror
set ytics 0,delta/2,1 out nomirror

set format x "%.2f"
set format y "%.2f"

set size ratio 1

unset key

fName="test.dat"

load sprintf("<gawk -v d=%f -f parse.awk %s", delta, fName)
plot fName u 1:2:3 w labels tc rgb 'white'

This script assumes the presence of auxiliary gawk script parse.awk in the same directory:

{
    printf "set object rectangle from %f,%f to %f,%f fc rgb fn(%d) fs solid\n",
        $1-d/2, $2-d/2, $1+d/2, $2+d/2, $3
}

This scripts accepts the required square size (-v d=%f in the invocation of gawk) and generates for each point a statement generating the corresponding square. These statements are consequently executed by the load command.

Mapping of the colors is done via the function fn defined in the main Gnuplot script. It takes the passed value and generates a rgb value which is then used with fc rgb in the rectangle specification.

Together, this then produces:

Discrete heat map with GNUPLOT

2 Answers