The `matrix` keyword (without a sequent `binary` keyword) in
{s}plot 'a.dat' matrix
indicates that data are stored in an ascii numbers matrix format.
The z-values are read in a row at a time, i. e.,
z11 z12 z13 z14 ...
z21 z22 z23 z24 ...
z31 z32 z33 z34 ...
and so forth.
In 3D, the x- and y-indices of the matrix surface plot correspond to column and row indices of the matrix, respectively, being enumerated from 0. You can rescale or transform the axes as usual for a data file with three columns by means of x=$1, y=$2, z=$3. For example
splot 'a.dat' matrix using (1+$1/100):(1+$2*10):3
A blank line or comment line ends the matrix, and starts a new surface mesh. You can select among the meshes inside a file by the index option to the `splot` command, as usual.
See `matrix` for examples of plotting rows and columns of the matrix in a 2D plot.