Bug with plot of a non-monotonic Series
Plotting a Series, whose index is not monotonic, gives wrong plot.
The following example gives markers at 4 different positions, whereas it should be 2 different positions.

```
In [168]: x1
Out[168]: 
2012-07-03 17:16:07+00:00    1.784596
2012-07-03 15:42:00+00:00    1.786576

In [169]: x1.sort_index().plot(style='g^')
Out[169]: <matplotlib.axes.AxesSubplot at 0x12fb2cc0>

In [170]: x1.plot(style='bv')
Out[170]: <matplotlib.axes.AxesSubplot at 0x12fb2cc0>

In [171]: plt.show()
```
