parallel_coordinates incorrect auto legend
Just tried the latest version of the parallel coordinates plot, but the default legend appears to be incorrect: instead of just choosing the four expected groups, one legend entry is included per row, many of them set to "None".

To reproduce, follow steps in the [pandas guide](http://pandas.pydata.org/pandas-docs/stable/visualization.html#parallel-coordinates).
## System info:

 Arch Linux 64-bit

 pandas 0.9.1rc1 (git)
 numpy 1.7.0b2
 matplotlib 1.2.0
## Screenshot

![pandas screenshot](http://i49.tinypic.com/33u7pjt.png)
