Decoupling contours and zrange

[Dima K. <gn...@di...>]

Hi. I'm trying to make a plot that's not working because I'm running
into some assumptions gnuplot is making. Help sought.

I want to make a 3D plot that contains contours on the xyplane AND ALSO
contains other data (points, say) in the xyz space. The contours are
rendered on the z=0 plane, so from the plot viewer's perspective this is
unrelated to the z axis in the plot. So I want the data on the z axis to
be decoupled from the data used to make the contours.

Example. I can make contours from a sinusoid:

  set contour base
  set cntrparam levels incremental -1,0.1,1
  set samples 100,100
  set isosamples 100,100
  splot [-5:5][-5:5] '++' using 1:2:(sin($1)*cos($2)) with lines nosurface

This works great. It generates a 3D plot with the contours rendered on
the z=0 plane only. Let's pretend I have other data plotted also, and
for whatever reason I also want to

  set zrange [0:0.5]

This breaks the contours! It looks like the zrange limit is applied to
the data before generating the contours, which sounds counter-intuitive
to me. Any suggestions about how to decouple this? Can we always use
zrange [*:*] for the purposes of contour generation? Should we?

Thanks