setup.py dies when mathplotlib-0.81 is compiled because either dev-python/numeric, dev-python/numarray or both are not installed on the system. Please update DEPEND settings in the ebuild.
According to the manual, those deps are not required, altought highly recommended. I've fixed the dependencies for matplotlib-0.81, so all docummented dependencies are required (including numeric and numarray). In the future we may USE flag both as "numeric".