secondary_y plot shows in separate figure
Hi, don't know if this is my Python/some module version.. but when I try to do the following, I end up with two figures popping up, instead of the expected one:

```
df[[0, 1]].plot(kind='bar', stacked=True, ylim=(0, 100))
df[[2]].plot(secondary_y=True, color='orange', linewidth=2.0)
```

The data in both plots looks right, the left/right y axes are correct etc.. but they're not on the same figure :(

Also if this is of any help, if I do `fig = plt.figure()` before running the commands above, I end up with another completely empty figure shown to me first.. 

Sorry if this is something stupid and/or not caused by pandas..
