i had the same issue. qt5 looks just too good on a mac retina screen, so downgraded matplotlib.
pip2 uninstall matplotlib
pip2 install matplotlib==2.0.0
as suggested above
or try upgrading to 2.1.2. (The release notes say “fixes a recursive draw bug and a major performance regression in Qt5Agg backend”.)
Seems to work on my machines (0.6.2, Win 10)
thanks @Paul_Soderlind that’s good to know. my strategy is, however: don’t touch PyPlot while it’s working 
so will leave at that for now.