I don't think the error is with matplotlib. That error should really just be a warning. This seems to be the real error: "[Wed Sep 15 14:03:17 2010] [error] [client 127.0.0.1] File does not exist: /var/www/django/grafico, referer: http://localhost/simuladores/index/
As for your imports, you really should consolidate and simplify it. The reason for importing only certain things is to prevent importing too much code and to keep the namespace clean. However, once you do "from matplotlib import use", python still has to process all of matplotlib. So, either import everything you need at that line or work from the matplotlib namespace (import matplotlib as mpl).
Also, I would be cautious about using both pylab and matplotlib.pyplot in the same code. These modules each make different assumptions about how you code and mixing them can cause some odd behaviors.
I hope this helps,