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From: Karl Ratzsch <mail.kfr@gmx.net>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: Samples, isosamples, and how they affect contour lines
Date: Thu, 12 Jul 2018 17:18:23 +0200
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Am 12.07.2018 um 08:50 schrieb nawridlz@gmail.com:
> By y-grid you mean the plane z = 80?
> 
no, the sampling distance between points in direction y.

   set sample 6	
   set isosample 500

gives a grid with 6 points in direction x, times 500 in direction y.
That's the grid you see with

   set view map
   splot $surface

How should the contour routine know what the z values at x=0 are when
there is nothing to interpolate between?

Actually your contour is terrible between [-10:-6] and [6:10] too. It
looks nice and round, but is a very sketchy interpolation, much worse
even in [-6:-2] and [2:6]

  reset
  set samp 100
  set isos 100
  set table $surf200 # this is a well-gridded plot
  splot x**2+y**2

  set samp 6
  set isos 500
  set table $surface # this isn't
  rep
  unset table

  set view map
  set cntrparam levels discrete 20, 40, 60, 80
  set contour
  unset surface   # comment out to see the grid
  splot $surf200 w l title "200x200 grid", \
        $surface w p ps .5 pt 1 title "6x500 grid"