Plotting DataFrame with descending index
I don't have time to dig into this at the moment, but I didn't want to forget about it. As you can see in the screenshot below, the points are being plotted in reverse x order (the blue line should be lower on the left). If I call matplotlib's plot (`plot(lc.index, lc.A_mean)`) it works correctly.

pandas 0.10.1, mpl 1.2.1, python 3.3

![screenshot from 2013-05-31 14 38 30](https://f.cloud.github.com/assets/327925/591033/03afa1dc-c9f8-11e2-91d1-671981029c28.png)
