I also ran into this annoying problem and it is indeed a problem in matplotlib (see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=502134), which is already fixed, but the new version has not yet reached Lenny.
As a workaround, you can install the version of matplotlib from unstable (http://packages.debian.org/sid/python-matplotlib) with "dpkg -i python-matplotlib...." as root. Thanks for reporting this anyway! Eike
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