Finding peaks with TH2::ShowPeaks()

I want to find the nr of peaks in a 2D histogram. I’ve used ShowPeaks and also some simple version with TSpectrum. What I want is to identify 2 peaks in the attached figure, which should be possible as there are clearly two humps? However I only get one peak.

If anyone can enlighten me on the meaning of the “sigma” and “threshold” parameters to the ShowPeaks() function I’d be grateful (I read about them but don’t really understand).

EDIT: uploaded a nicer-looking picture. Note that most bins are zero.
cluster.pdf (13.6 KB)