BUG: enabling subplots works unexpectedly
Found 2 minor issues related to plotting with `subplots=True`.
1. If `DataFrame` only has single column, plot raises `TypeError`. It is because `_subplots` returns `Axes` instance, not array.
2. If `kind=scatter` or `kind=hexbin` is specified with `subplots=True`, additional blank axes is drawn.
