3D Data
Gnuplot support two file formats for 3D data to work with splot. This article is adapted from Gnuplot in Action.
Usually, 3D plotting does not make much sense since it cannot really provide insights about quantities, and we would like to create its 2D projection (such as heatmap).
Grid format
If the data file is in grid format, each line must contain both x and y coordinates, as well as the z value that is to be plotted. Data must be organized into data blocks, which are separated from each other by a single blank line. Each block must contain all data points for a single row of data points.
<x0> <y0> <z(0, 0)>
<x0> <y1> <z(0, 1)>
<x0> <y2> <z(0, 2)>
...
<x0> <yN> <z(0, N)>
<x1> <y0> <z(1, 0)>
<x1> <y1> <z(1, 1)>
...
splot "grid" u 1:2:3 w lines
Matrix format
The matrix format1 is an alternative file format for data on a regular, rectangular grid. It’s more compact, because it doesn’t store the (redundant) information on x and y coordinates for every data point—only the z values at the grid locations are kept.
splot "matrix" matrix with lines
Here we need to specify its format as matrix explicitly.
1 There are two permissible matrix formats, and we only focus on a compact one.