No more label rotation when secondary_y=True on the same data
Pandas version I am using: 0.16.2

My initial chart is produced like this: 

```
combined['fraud_count'].unstack(-2).unstack(-1).fillna(0).plot()
```

which looks very neat and perfect, as expected.

In the next step, I am trying to use the secondary_y feature like this: 

```
combined['fraud_count'].unstack(-2).unstack(-1).fillna(0).plot()
combined['fraud_amount'].unstack(-2).unstack(-1).fillna(0).plot(secondary_y=True)
```

which doesn't produce a secondary axis. So, I do it this way: 

```
ax = combined['fraud_count'].unstack(-2).unstack(-1).fillna(0).plot()
ax2 = combined['fraud_amount'].unstack(-2).unstack(-1).fillna(0).plot(secondary_y=True, ax = ax)
```

which produces the chart I want. 

**However, the labels on the x-axis are not rotated any more but written horizontally and overlapping each other.**

I was trying some ways to get this fixed but none of them really seemed to work. 

I believe this is a quite common thing to do, so I'd be happy to hear how this could be done. Any help is welcome!
