BUG: strange timeseries plot behavior
I added a test with the simple repro at the top of bug #6608, which appears to be the missing piece for the bug to be closed.

I went back in time to a previous version of pandas, and I confirmed the issue.
```
Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> import datetime
>>>
>>> s1 = pd.Series([1, 2, 3], index=[datetime.datetime(1995, 12, 31), datetime.datetime(2000, 12, 31), datetime.datetime(2005, 12, 31)])
>>> s2 = pd.Series([1, 2, 3], index=[datetime.datetime(1997, 12, 31), datetime.datetime(2003, 12, 31), datetime.datetime(2008, 12, 31)])
>>>
>>> ax = s1.plot()
>>> s2.plot(ax=ax)
<matplotlib.axes._subplots.AxesSubplot object at 0x04ECED10>
>>> s1.plot(ax=ax)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tools\plotting.py", line 1829, in plot_series
    plot_obj.generate()
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tools\plotting.py", line 905, in generate
    self._make_plot()
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tools\plotting.py", line 1317, in _make_plot
    self._make_ts_plot(data, **self.kwds)
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tools\plotting.py", line 1388, in _make_ts_plot
    _plot(data, 0, ax, label, self.style, **kwds)
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tools\plotting.py", line 1372, in _plot
    style=style, **kwds)
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tseries\plotting.py", line 82, in tsplot
    left, right = _get_xlim(ax.get_lines())
  File "C:\Projects\Sprints\PandasVirtualEnvLatest\PandasVirtualEnvLatest\oldpy27\lib\site-packages\pandas\tseries\plotting.py", line 226, in _get_xlim
    left = min(x[0].ordinal, left)
AttributeError: 'datetime.datetime' object has no attribute 'ordinal'
>>> print(pd.__version__)
0.13.1
>>>
```
