I have a strange problem. It seems to me it is related to
mpl.axes.Axes.add_collection(), but I am too lost to be sure.
Well, I haven't solved this problem yet, but it seems that the add_collection()
method is not the one at fault. It looks like the lasso widget I am using to
selct the regions is returning for the fourth region the same vertices than for
the third one. That's why I saw nothing for the fourth region when I was
actually passing the add_collection() method a valid PolyCollection. I'll keep
on looking... but why the fourth?