Add proxy artist to Axes.fill_between[x] for label support (Issue #1341)

[ycopin]

I guess the true problem in Issue #1341 is that Axes.fill_between[x] returns a PolyCollection which does not support labels.

[leejjoon]

Thank you for the PR. Unfortunately, I don't think this is a good solution for the problem. I think it is best to implement a legend handler for the PolyCollection class (or for fill_between command).

The link below may give you some idea how legends are handled.

http://matplotlib.org/users/legend_guide.html#legend-of-complex-plots

Also, check out the errobar or box command (and corresponding handler routines) for actual implementation.

[ycopin]

Thanks for the link, I'll look into it (time permitting).