Greetings,
I was able to make a small change to axes.py which augments errorbar() with a
keyword, 'ealpha' (similar to 'ecolor'), which controls the alpha blending of
the error bar lines. The motivation was to make overlapping error bars of
different color slightly less confusing (or at least more interesting). It's
up to the user to resist the temptation to utilize it for "error
reduction." 
Patch attached, use ad lib.
Mike
errorbar_alpha.patch (1.42 KB)