An isoline is a curve parametrized by one of the surface parameters
while the other surface parameter is fixed. Isolines are a simple
means to display a surface. By fixing the u parameter of surface
s(u,v), the iso-u lines of the form c(v) = s(u0,v) are produced, and
by fixing the v parameter, the iso-v lines of the form c(u) = s(u,v0)
are produced.
The isoline density of surfaces may be changed by the set isosamples
command. By default, sampling is set to 10 isolines per u or v axis.
A higher sampling rate will produce more accurate plots, but will take
longer. This parameter has no effect on data file plotting.
Syntax:
set isosamples <expression>
show isosamples