While trying to recreate this outside of my work code, I found that I cannot do it in a simple manner- that is, just creating a ton of points and plotting them doesn't reproduce the problem.  I'm going to revisit this and make sure it isn't a problem in my working program versus what I thought it was, e.g. some sort of drawing limitation.

FYI, I get this using the Qt4Agg backend and am using MPL 1.0.1