Plotting FreeFEM++ data to GNUPLOT

Question

I'm trying to plot data obtained with FreeFEM++ using gnuplot.

From FreeFEM I have saved the data with the following code:

{ ofstream ff("sol.dat");
    for (int i=0;i<Th.nt;i++){
            for ( int j=0; j<3; j++){
                    ff<<Th[i][j].x << "\t" << Th[i][j].y << "\t" << u[][Vh(i,j)] << endl;
            }
            ff<<Th[i][0].x << "\t" << Th[i][0].y << "\t" << u[][Vh(i,0)] << "\n\n\n";
    }

}

A sample of the first lines of data is:

0.635787    -0.0440338  0.0056924
0.65234 -0.063181   0.00797757
0.655369    -0.0412323  0.00795786
0.635787    -0.0440338  0.0056924


0.597634    -0.0376 3.06323e-33
0.613904    -0.0585366  0.0030425
0.616879    -0.0388107  0.0030295
0.597634    -0.0376 3.06323e-33

where for some reason which I don't know there is a data point repeated two times in each 'pack'. But this is how they tell you to do it in the FreeFem manual. Apparently GNUPLOT needs the spaces between 'packs of data' for some reason.

I'm trying to plot the data with the gnuplot script:

set pm3d at b
set palette rgbformulae 30,31,32
set hidden3d
splot "sol.dat" with lines palette

It plots fine but I think pm3d is not doing anything. I thought pm3d would include the colored pattern of heat intensity in the bottom of the plot. Also I don't find the way to fill with solid color the gaps between the lines in the plot.

OUTPUT SAMPLE

Thank you.

You are mixing several things (pm3d, hidden3d, lines palette), look like you want a very fancy plot... You must separate two data blocks with one row in order to work with pm3d. The correct syntax is set pm3d at b. You need to add one point twice, because pm3d uses four points to create one rectangle with one color. A 4x4 matrix would result in 3x3 colored rectangles. Could you explain in words, how your plot should look like (how are the lines to be displayed?, how the surface?, pm3d only on the base? what should be hidden etc). As you have it, the options don't fit together. — Christoph
@Christoph, thank you. I have fixed the typo and included a better description of what I want and an image of what I'm getting. I did not know those commands are not compatible. — Janus Gowda

Christoph Christoph · Accepted Answer · 2013-10-25T18:52:41

An answer to this requires first a proper explanation of different possibilities to organize the data:

Two empty rows separate two different data sets. These aren't connected at all, no lines are drawn between them. In your case this is required, because gnuplot doesn't support the kind of grid you have for a single surface.
pm3d works only within a single data set, and needs a regular grid (see the pm3d demos). Two lines (isolines) of a single surface must be separated by only one empty row.

Still you have an irregular grid, which pm3d can't handle. Inside gnuplot you can use dgrid3d to resample your input data to get a regular grid and plot that one with pm3d.

But dgrid3d affects all data files of a one splot command. So you'll also need multiplot to use two splot commands.

The following script shows how it could work, but as I don't have the full data set and don't know how dgrid3d copes with that many data sets (see 1. above), this is only a very rough guide:

set multiplot

set pm3d at b
set dgrid3d 200,200
unset key
splot 'sol.dat' nosurface

unset dgrid3d
unset pm3d
splot 'sol.dat' with lines palette

unset multiplot

That should work, but you probably need to tune the dgrid3d call. Also, some other enhancements may be required (plotting the border, tics and colorbox only once etc.)

Plotting FreeFEM++ data to GNUPLOT

1 Answers