Series.hist() potential bug.
The following line of code prints the wrong distribution:

``` python
import matplotlib.pyplot as plt
import pandas as pd

# df is a pd.DataFrame object.  Define df here...

plt.figure();
(df['x1'] - df['x2']).hist(normed=True)
plt.show();
```

It should print the green distribution, but it prints the blue instead (which is not even a proper distribution).  

I'm including the exact lists I used, x1 and x2, below.  These are ordered lists, so please insert them into your DataFrame accordingly.

(Pardon my presentation of the x-labels in the green figure.  Just shift them 1/2-bar length to the right.)

``` python
x1 = [4, 4, 6, 4, 4, 4, 4, 5, 5, 6, 4, 4, 5, 6, 4, 4, 4, 4, 5, 5, 6, 5, 3, 6, 6, 7, 7, 7, 4, 6, 4, 5, 4, 5, 4, 3, 7, 7, 7, 5, 4, 5, 4, 3, 5, 4, 4, 5, 6, 5, 4, 4, 4, 5, 6, 4, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 6, 7, 6, 7, 7, 7, 7, 6, 7, 7, 7, 5, 7, 7, 7, 7, 7, 5, 7, 5, 5, 7, 6, 7, 6, 4, 6, 7, 6, 7, 1, 6, 7, 2, 7, 4, 6, 7, 6, 6, 4, 6, 7, 7, 7, 5, 7, 6, 7, 4, 6, 7, 7, 6, 5, 7, 6, 6, 6, 6, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 4, 7, 4, 5, 7, 6, 7, 4, 5, 7, 7, 7, 7, 7, 7, 7, 7, 4, 4, 5, 5, 7, 7, 7, 7, 6, 7]

x2 = [4, 4, 6, 3, 4, 4, 4, 5, 6, 6, 4, 4, 5, 6, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 7, 7, 7, 5, 7, 2, 4, 4, 4, 6, 7, 7, 7, 7, 4, 5, 4, 5, 3, 5, 4, 5, 5, 6, 5, 5, 5, 4, 5, 6, 4, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 4, 7, 7, 7, 7, 7, 7, 7, 5, 5, 7, 7, 7, 5, 5, 5, 7, 7, 7, 7, 7, 7, 7, 7, 7, 6, 5, 6, 6, 5, 6, 7, 7, 7, 5, 7, 7, 6, 5, 5, 6, 6, 6, 5, 7, 5, 5, 5, 5, 4, 4, 3, 4, 7, 4, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 6, 6, 4, 4, 6, 5, 6, 4, 5, 7, 5, 7, 7, 7, 7, 7, 7, 5, 5, 7, 7, 7, 7, 7, 7, 7, 7]
```

![figure_correct](https://cloud.githubusercontent.com/assets/1490869/11700208/7aaacb3e-9e96-11e5-8e9a-4146ed81f2ee.png)

![figure_incorrect](https://cloud.githubusercontent.com/assets/1490869/11700205/77e43138-9e96-11e5-8cf4-98458b0452db.png)
