plot with unsorted indices misses data points
Compare these 2 plots below, this is not expected behavior:

```
import pandas as pd
from numpy.random import rand
testarr1=rand(100)
testarr2=rand(100)
pd.DataFrame(testarr2,index=testarr1).plot(style='.')
pd.DataFrame(testarr2,index=testarr1).sort().plot(style='.')
```
