df.plot produces wrong result when x vals are unsorted. (Dupe #2726)
Could be related to #2923. The following code produces two different plots for me in pandas 10.1.

```
import pandas
from matplotlib import pyplot
from numpy.random import randn

data = pandas.DataFrame(randn(6, 2))
pyplot.plot(data[0], data[1])
data.plot(0, 1)
```

One can actually implement a test for this by comparing the collections produced by the plot. 
