Plot two datasets on the same graph with gnuplot. One with dgrid3d, the other one without

Question

I'm trying to plot two data sets with gnuplot. They are both (x, y, z) triplets. They are not arranged on a grid. I want to plot one of them using dgrid3d and pm3d. On top of that I want to overlay the other data set but as just scattered points.

To give a more concrete example: I am trying to plot the effect of a cylinder approaching a surface. I want to plot the response of the surface and that's where dgrid3d comes in handy. On top of that, I want to plot the position of the cylinder and I have its circumference as points.

I used:

set dgrid3d 100,100,4
set pm3d
splot "dataset1" with pm3d, "dataset2" with dots

The data set has about 100x100 points, arranged on a near-square, so 100,100 works best here. No matter how I plot the second data set, it always ends up being a square of the same dimensions as the cylinder, instead of a nice circle. When I turn dgrid3d off, I can plot the second data set on its own and the result is a nice circumference of the cylinder.

So my question is: is it possible to plot a 3D graph using two data sets, one using dgrid3d and the other one not using it?

Answer 1

Yes, this is possible, but it is a little more tricky than you might think. The key is to use set table

For your example:

set dgrid3d 100,100,4
set pm3d explicit
set table "interpolated_data.dat"
splot "dataset1" with pm3d  #writes the interpolated data to "interpolated_data.dat"
unset table
unset dgrid3d
splot "interpolated_data.dat" with pm3d, "dataset2" with dots

The reason that your attempt didn't work is because when dgrid3d is in effect, All data read in is interpolated to a grid and then plotted using whatever style you specify.

From gnuplot's help dgrid3d When enabled, 3D data read from a file are always treated as a scattered data set.

As a side note, this method can also be used to plot contours on top of a pm3d as well.