BUG: plot with kind=scatter fails when checking if an array is in the DataFrame
``` python
df = pd.DataFrame({'A': [1, 2], 'B': [3, 4]})
c = np.array([1, 0])
df.plot(kind='scatter', x='A', y='B', c=c, cmap='spring')
```

fails with a TypeError.

we check `elif c in self.data.columns` which tries to hash the array.

(I know I can just add the array as a column to the DataFrame)
