Update plotting.py
Should fix https://github.com/pydata/pandas/issues/10214#issuecomment-106091734

Thanks @scls19fr !

Can you add:
- test cases (pls see `test_graphics.py`)
- release note for the bug fix (`doc/source/whatsnew`)
 Sorry @sinhrks but I feel:

![engrenage](https://cloud.githubusercontent.com/assets/109167/7885929/0272a112-0627-11e5-9696-e18f572e2045.jpg)
 In `pandas/doc/source/whatsnew/v0.17` there's a bunch of one-line notes describing bug fixes. Add one like that.

Tests for this are in `pandas/tests/test_graphics.py` The idea is to make a test that fails without your change, but works with your change. There's a bunch of helpful info here: https://github.com/pydata/pandas/wiki/

Let us know what you're stuck on, and we can walk you through it. Most of us are in the [chat room](https://gitter.im/pydata/pandas) if you want quicker feedback on git stuff.
 Superseded by https://github.com/pydata/pandas/pull/10233
