df.plot(x=..., y=...) fails for mixed-integer column names
``` python
In [187]: df = pandas.DataFrame(np.random.randn(100,3),columns=['A', 'B', 0])

In [188]: df.head()
Out[188]: 
          A         B         0
0  0.699932 -1.090587  1.014325
1 -0.148856 -0.994177 -1.107741
2  1.384710  0.107488  1.108757
3 -0.795969 -1.297598 -0.268348
4 -0.247695 -0.627048  0.010415

In [189]: df.plot(x='A', y=0)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
...
KeyError: 'no item named A'

In [190]: 
```
