Ternary

Hi-

I used your work on ternary plots back in 2011, and found it very useful.

I just today tried to produce another ternary plot, but found that your code no longer worked against recent versions of matplotlib.   I spent a few hours tracking down the problem (the methods that needed to be overwritten in Transform apparently changed slightly); now your ternary_demo.py seems to behave the way one would want, at least.

I've made the necessary changes to my fork of matplotlib.  May I suggest that you pull it, in case others could benefit from this nice projection?

It'd be great if your work could also be merged into upstream, of course! 

-Ethan Ligon