Problem with

The following code (is a simple modification of

    > produces a stock chart. SOme of the lines
    > are thicker than the others. All the lines should have the
    > same width.

This is a little bug that crops up frequently because of how agg
handles subpixel rendering. I added a little hack to the line
collection code to "snap to pixel center" for len(2) lines. Should
fix your problem.

Use CVS revision

Checking in src/_backend_agg.cpp;
/cvsroot/matplotlib/matplotlib/src/_backend_agg.cpp,v <-- _backend_agg.cpp
new revision: 1.89; previous revision: 1.88

or later.