Pandas groupby plot problem
In Pandas 0.162 I used:

```
df[df['var']==True].groupby('var2').plot(ax=ax[1],x='dfcolumnx',y='dfcolumy',legend=False,use_index=False ,kind='line',style='o-')
```

this plots df['dfcolumx'] as the x-axis and df['dfcolumy'] as the y-axis on ax[1].

This is not true for pandas 0.17:

This gives the wrong plots: instead of plotting in ax[1] it creates N new plots with un-understandable x and y axis and line plots. 
