It is just a matter of the axis range. Your radial axis goes between 40 and 80, but actually your values of r go below 40, so they jump to the negative side.
Just call: vgP[0] -> GetPolargram()->SetRangeRadial(0,100);
By the way, you may be interested in this code attached here, which does something similar to what you want, i.e. the Klein-Nishina cross section and Compton edge analysis, but in normal histograms. Should not be a big deal to convert to polar graphs. See http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-204988 for details on the notation.
That it is!!
Best answer ever.
Thank you very much ferhue.
By the way, I do not understand the ways how vgP[0] → GetPolargram()->SetNdivRadial(10); works according to the doxygen
Set the number of radial divisions: enter a number ij with 0<i<99 and 0<j<99.
i sets the major radial divisions.
j sets the minor radial divisions.
So that I was expecting 1 major division and no minor divisions.
But I am not getting this.
The documentation is not very clear at this point. It should rather say:
Enter a number N = i*100 + j, where i (0-99) are the major radial divisions and j (0-99) are the minor ones. So j should occupy always 2 digits.
I know this should be in another topic, but in any case.
Do you know how to import UTF8 in titles. I need to use some Spanish accents (tildes) but
from all the available accents here https://root.cern.ch/doc/master/classTLatex.html
the most similar is #acute, again this can be esthetics, but the way those accents looks are not so good. Even in the same example they provide you can realize it.
yes, using #acute seems to be the easiest way. If you need better accents, I suggest that you save the canvas as .tex. Then, you compile with latex/tikz/pgf, where UTF8 is supported without problems.
Another way might be to use the Qt/Root plugin. See for example: