BUG: plot(kind='bar') ignores use_index keyword

[TomAugspurger]

with df = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd']).

From df.plot(kind='bar', use_index=False):

From df.plot(use_index=False):

Most of the time it doesn't make sense to do a bar plot and ignore the groups formed by the index, but I had an index with a bunch of long strings and just wanted to see the distribution (pretty much a histogram), so I didn't care about the index.

PR inbound.