API/BUG: inconsistent plotting of new CategoricalIndex
To record the issue we discussed yesterday, to be solved for 0.16.2

Plotting of categorical:
- y-axis: the 'values' itself are used
- x-axis: they are not regarded as values, but as all unique items that just gets represented by a `range(len(cat))` x data

Overview: http://nbviewer.ipython.org/gist/jorisvandenbossche/992d9d34dbfcfd8bc326

Disclaimer: didn't yet look into the code to see why this is like this.

Way forward:
- for now: handle CategoricalIndex the same as column with Categorical values (so use the values itself)
  - this will also mean that a CategoricalIndex with string categories will raise now (in 0.16.1 it did not but plotted all values just in order, also discarding the fact the values with the same category should be regarded as equal)
- later, we can try to implement more fancy / intelligent categorical plotting (there are already #9069 and #8712 about this)
