DENSITY:random_points
If v is a vector, x and y are assumed to represent a scattered set
of points, where v is the altitude corresponding to the location
(x,y). A regular grid, or matrix, is interpolated on these
scattered points by means of a Thiessen triangulation of the plane.
If v is a matrix, x and y default to [1;2;3;...]. If two vectors, x
and y, are entered with the matrix v, the length of x must be greater
than or equal to the number of columns of v and the length of y must
be greater than or equal to the number of rows of v.
The optional scalars p1 and p2 can be used to broaden or shrink the
range of data values, from the full data range, min to max. If v is
a data value and if v < p1*(max-min)+min, that data value is treated
as min. If v > p2*(max-min)+min, that data value is treated as max.
The default values are: p1 = 0 and p2 = 1.
If the qualifier \COLOUR is used, the range of values of the
matrix is divided into eight equal levels and a different colour
is associated with each level. The point is drawn with the colour
corresponding to the relative size of the interpolated value.