Mismatch of pm3d (color) & line 3D surface

Trying to plot a non-grid 3D dataset, I've used dgrid3d 960,960 to capture the many fluctuations of the z-values. Then I've plotted them with pm3d. To overlap it with the same 3D plot but with lines, I must reduce the parameters of dgrid3d to, say, 120,120 so that they may be individually visible. In principle that's an easy endeavor,

set dgrid3d 960,960
splot "FILE" w pm3d notitle
set dgrid3d 120,120 qnorm 9
rep "FILE" w l

The result displays a moderate mismatch between both plots,

I guess that's because set dgrid3d 120,120 renders different z-approximated values than set dgrid3d 960,960. If I keep 960,960 for the plot with lines, then you can imagine... it comes all full of lines, 960 in each direction. And set isosamples doesn't work with dgrid3d. I wish there was something like rep "FILE" w l [120,120] but there is no. Any solution ?

This is not a full solution to your problem since it remains true that if you change the dgrid3d settings then the generated surface will also change. I didn't really notice it strongly in the image you posted,but it is very clear in the figure below. It does show how you can achieve your wish to change the gridding mid-stream while plotting. I take it that what you really want, however, is to keep the gridding the same but only draw every 10th line or so. That seems like a reasonable thing to do but I don't think it is currently possible in gnuplot.

Note: This technique requires gnuplot verion 6!

Several things to note in this script.

• hidden3d does not mix nicely with pm3d. You can work around this by opting out of the hidden3d processing for the pm3d surface (nohidden keyword) and forcing the entire hidden3d part of the plot to appear in front of the rest (set hidden3d front).
• The function block mechanism introduced in version 6 allows you to embed gnuplot commands in a function, even if those commands are not something you would normally be able to execute in the middle of a plot command. We use it here to define a function that changes the gridding parameters to dgrid3d.
• Every plot or splot component accepts an optional definition A=B before the item being plotted. We use this here to call our function block with new gridding parameters for each component of the plot.

Script:

set palette defined (0 "dark-blue", 1 "white")
set xrange noextend
set yrange noextend
set zrange [-0.6 : 1.6]
set xyplane 0
unset key

set pm3d noborder explicit
set hidden3d front

function $set_grid(N) << EOF
    set dgrid3d N,N qnorm 9
EOF

splot A=$set_grid(480) "DATA" with pm3d at bs nohidden, \
      A=$set_grid(120) "DATA" with lines lw 1.0 lc "black"

Result:

That said, you may get an acceptable result by making the grid not quite so fine and the lines in the second component of the plot thinner. Here is the same plot drawn with

splot A=$set_grid(480) "DATA" with pm3d at bs nohidden, \
      A=$set_grid(480) "DATA" with lines lw 0.2 lc "black"