BUG: scatter plot with categorical data raises KeyError
```python
df = pd.DataFrame({'x':[1,2,3,4], 'y':pd.Categorical(['a', 'b', 'a', 'c'])})
df.plot(x='x', y='y', kind='scatter')
```

raises `KeyError: 'y'`, while the column certainly exists, which can be very confusing. 
Without the scatter (just `df.plot(x='x', y='y')`), it raises the more informative `TypeError: Empty 'DataFrame': no numeric data to plot`