Hexbin plots does not display x label and xtick labels
I'm currently using python 2.7, pandas 0.16.2.dev, and ipython version 3.1.0

In brief, it seems that there are cases that the sharex option of pandas.DataFrame.plot(kind='hexbin') defaults to True instead of False. This causes the x label and xtick labels to be invisible.

![pandas-sharex-bug](https://cloud.githubusercontent.com/assets/9617231/8895078/7bf75cc4-3392-11e5-94b8-573d3ca68eea.png)

Preceding stack overflow and ipython github threads that led to here:

http://stackoverflow.com/questions/31220329/pandas-dataframe-hexbin-plot-has-no-xlabel-or-axis-values

https://github.com/ipython/ipython/issues/8653
