BUG: plot(kind=hist) results in TypeError if it contains non-numeric data
```
df = pd.DataFrame(np.random.rand(20, 5), columns=['A', 'B', 'C', 'D', 'E'])
df['C'] = ['A', 'B', 'C', 'D'] * 5
df['D'] = df['D'] * 100

df.plot(kind='hist')
# TypeError: cannot concatenate 'str' and 'float' objects
```
