You must install numpy or scipy before installing matplotlib. If you do not want to do it yourself you cand download Scipy superpack "This shell script will install recent SVN builds of Numpy (1.3) and Scipy (0.7), as well as Matplotlib (0.98), iPython (0.8.3) and PyMC (2.0 beta) for OS X 10.5 (Leopard) on Intel Macintosh."
http://macinscience.org/?page_id=6 -- http://mail.python.org/mailman/listinfo/python-list
