pandas diff().hist() produces inaccurate plot
Using a df.diff().hist method creates a different (and inaccurate) multiple plot than using the matplotlib api.
My data are Super Rugby differential scores. The matrix of data is below.

``` python
#Create base matrix of score differentials across Teams (row) x Round (col)
S15_Base = np.matrix([[-10,6,-24,0,28,43,-12,42,-5,-17,14,-6,41,0],#Crus
                       [10,-10,-5,0,4,-4,-24,8,0,5,-2,1,20,-17],#Reb
                       [44,-2,5,12,29,-15,0,17,-2,-5,13,-3,-1,10],#Brum
                       [-44,12,-7,-18,-29,0,-1,-8,-21,1,-16,0,-41,17],#Red              
                       [-14,-17,-3,3,-28,4,1,4,2,0,5,-2,5,-10],#Lion              
                       [14,4,29,0,7,7,24,5,0,-5,16,6,8,4],#Hurr
                       [-5,-11,-1,-3,-7,0,-12,-7,2,-6,-14,17,-20,5],#Blues
                       [5,2,24,-3,9,-1,10,7,0,17,8,-1,0,-4],#Chiefs
                       [-6,17,-8,-16,17,1,6,-42,-2,-7,0,-33,-8,-15],#Sharks
                       [6,0,1,-19,-17,-43,-10,-17,9,-1,-5,8,0,-21],#Cheetahs
                       [-12,-4,8,19,0,1,12,-4,21,7,-2,2,0,-5],#Bulls
                       [12,11,3,16,-9,0,-18,-5,14,7,2,-8,1,0],#Stormers
                       [-12,10,0,18,-7,15,12,0,-14,5,2,3,-7,15],#Waratahs
                       [12,-12,-29,-12,-4,-1,-6,0,-9,-7,-8,-17,7,0],#Force
                       [0,-6,7,3,7,-7,18,0,5,6,-13,33,-5,21],#Highlanders
                       ])
```

I create a dataframe with:

```
import pandas as pd
s15df = pd.DataFrame(S15_Base.T, columns=['Crusaders', 'Rebels', 'Brumbies', 'Reds', 'Lions', 'Hurricanes', 'Blues', 'Chiefs', 
                                      'Sharks', 'Cheetahs', 'Bulls', 'Stormers', 'Waratahs', 'Force', 'Highlanders'])
```

The following call produces the multiple plots by team:

``` python
s15df.diff().hist(color='purple', alpha=0.5, figsize=(12,8),sharey=True,sharex=True,bins=20)
```

The result is different in at least one instance (for the Sharks, who never win by 60pts or more), yet have a bin reflecting this frequency. I checked using:
`s15df.Sharks.plot(kind='hist',bins=20)` and 
`fig = plt.figure()`
`ax = fig.add_subplot(111)`
`x = [s15df.Sharks ,s15df.Crusaders,s15df.Reds]`
`numBins = 20`
`ax.hist(x,numBins,color=['green','blue','red'],alpha=0.8)`
`plt.show()`

Both the latter two plots were identical and numerically correct, but different than the multiple plot. 

PS: Please forgive me if I err in any way posting this as an issue -- am a huge fan of pandas but novice in the issue-tracking thing! :)
