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(gnuplot.info)isosamples

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 The isoline density (grid) for plotting functions as surfaces may be changed
 by the `set isosamples` command.

 Syntax:
       set isosamples <iso_1> {,<iso_2>}
       show isosamples

 Each function surface plot will have <iso_1> iso-u lines and <iso_2> iso-v
 lines.  If you only specify <iso_1>, <iso_2> will be set to the same value
 as <iso_1>.  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.
 These parameters have no effect on data file plotting.

 An isoline is a curve parameterized by one of the surface parameters while
 the other surface parameter is fixed.  Isolines provide 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.

 When a function surface plot is being done without the removal of hidden
 lines, `set samples` (Note: samples )  controls the number of points sampled
 along each isoline;  see `set samples` and `set hidden3d`
 (Note: hidden3d ).  The contour algorithm assumes that a function sample
 occurs at each isoline intersection, so change in `samples` as well as
 `isosamples` may be desired when changing the resolution of a function
 surface/contour.