Some time ago I tried to install matplotlib using zc.buildout, to make
sure I always had the same version everywhere and so on.

Even if numpy was installed it was not found, and someone told me to
modify the setup.py of matplotlib.
But since I think it should work out of the box I think I'm doing
something wrong OR there is a bug.

It should be a known issue but I didn't find much about, if necessary I
can try again and paste the output...