MetaPost would be the usual suspect, but he probably used his own Python-to-PostScript wrapper: http://www.math.ubc.ca/~cass/piscript/docs/piscript.html .

Recently many mathematicians I know are plotting with NodeBox which is a VJ software akin to Proce55ing but using Python.

Edit: He also authored a lovely PostScript cook book http://www.math.ubc.ca/~cass/graphics/manual/