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  {,}
         show isosamples

 Each surface plot will have  iso-u lines and  iso-v lines.
 If you only specify ,  will be set to the same value as
 .

 When a surface plot is being done without the removal of hidden
 lines,  set samples also has an effect on the number of points being
 evaluated. See  set samples.