Plotting series (columns) fails
Calling plot from matplotlib with series (e.g. columns of a DataFrame) fails, because it tries to add an axis to the series objects. A traceback can be found here: https://gist.github.com/1281034 .

One way to solve this could be, if we try to index a series to multiple dimensions, simply return a plain numpy array of the data. Or maybe it makes more sense to make changes on the matplotlib side?

`Series.plot()` and `DataFrame.plot()` are designed to plot columns against the index, but don't appear to help with plotting one column against another.
