Plotting a dataframe with date unordered date index
Migrated from [StackOverflow](http://stackoverflow.com/questions/14075326/pandas-runtimeerror-in-tseries-convertor-when-plotting). The following gives an exception (when trying to plot a DataFrame with date indices). I'm using '0.10.0b1'.

```
In [1]: df = DataFrame(randn(3,1),index=[date(2012,10,1),date(2012,9,1),date(2012,8,1)], columns=['test'])

#the same bug is present using datetime rather than date
In [2]: df
Out[2]: 
                test
2012-10-01 -0.315065
2012-09-01  0.100678
2012-08-01 -0.230353

In [4]: df.plot() 
#error
```

Here is the traceback:

```
Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 1413, in __call__
    return self.func(*args)
  File "/usr/lib/python2.7/lib-tk/Tkinter.py", line 498, in callit
    func(*args)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 254, in idle_draw
    self.draw()
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_tkagg.py", line 239, in draw
    FigureCanvasAgg.draw(self)
  File "/usr/lib/pymodules/python2.7/matplotlib/backends/backend_agg.py", line 421, in draw
    self.figure.draw(self.renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/figure.py", line 898, in draw
    func(*args)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/axes.py", line 1997, in draw
    a.draw(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, **kwargs)
  File "/usr/lib/pymodules/python2.7/matplotlib/axis.py", line 1041, in draw
    ticks_to_draw = self._update_ticks(renderer)
  File "/usr/lib/pymodules/python2.7/matplotlib/axis.py", line 931, in _update_ticks
    tick_tups = [ t for t in self.iter_ticks()]
  File "/usr/lib/pymodules/python2.7/matplotlib/axis.py", line 878, in iter_ticks
    majorLocs = self.major.locator()
  File "/usr/lib/pymodules/python2.7/matplotlib/dates.py", line 750, in __call__
    return self._locator()
  File "/usr/lib/pymodules/python2.7/pandas/tseries/converter.py", line 317, in __call__
    (estimate, dmin, dmax, self.MAXTICKS * 2))
RuntimeError: MillisecondLocator estimated to generate 5270400 ticks from 2012-08-01 00:00:00+00:00 to 2012-10-01 00:00:00+00:00: exceeds Locator.MAXTICKS* 2 (2000) 
```

Note that it plots ok, using `df.sort().plot()`.
