BUG: hist plot secondary-y doesn't work
The labels overlap

``` python
In [1]: df = pd.DataFrame(np.random.randn(30, 4), columns=list('abcd'))

In [2]: ax = df.a.plot(legend=True)

In [3]: df.b.plot(ax=ax, legend=True)
Out[3]: <matplotlib.axes._subplots.AxesSubplot at 0x10938f6a0>
```

![hist](https://cloud.githubusercontent.com/assets/1312546/6541625/112de2b4-c4a2-11e4-8004-edb939520b04.png)
