DOC/BUG: Pandas Adds a Dashed Line in Bar Plot (unnecessarily?)
When using `df.plot(kind='bar')` pandas adds a dotted line to the x-axis (`kind='barh'`, line is added to y-axis). Why is the dotted line added ([line in the code](https://github.com/pydata/pandas/blob/master/pandas/tools/plotting.py#L1899))? 

This line is undocumented and there is no way to remove it without a hack. It'd be nice to have it documented (if it is necessary) or removed.
