I would like to invite anyone to take at look at this and would be glad about 
any comment or suggestion.

The module pyxmath.py included in Simply Draw contains additional 
functionality for PyX (two graph data sources and a graph style). Perhaps you 
may want to think about including similar functionality to PyX itself.

Concering the Python function stuff, I'm not sure whether this will be
needed in PyX 0.9 anymore (but it could still be useful). The reason for
this being that we removed our own expressions parser in favour of using
the Python eval function. Commenting on the graph style, I'll happily
leave to André...

You had an expression parser? Oh my. I always assumed  you used eval().