Log scale on bar chart hides bars
When I try use log scales on a bar plot in pandas, the area of the bar disappears. Example in [this ipynb](http://nbviewer.ipython.org/urls/gist.github.com/gjx/5298007/raw/d1a0bd774a253fa1fc1f12a005dd70a697e8f360/pandas_bar_log)

I think the answer is to pass `log=True` to `bar`, as in [this SO answer](http://stackoverflow.com/questions/14047068/python-pyplot-bar-plot-bars-disapear-when-using-log-scale), but when I add `log=True` as a kwarg to `Series.plot()`, it doesn't fix the problem.
