BUG: strange timeseries plot behavior
After some discussion below, here's a simple repro case:

``` python
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)])

# plot first series, then add the second series to those axes, then try adding the first series again
ax = s1.plot()
s2.plot(ax=ax)
s1.plot(ax=ax)
```

causes

```
Traceback (most recent call last):
  File "simple_repro.py", line 10, in <module>
    s1.plot(ax=ax)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 2116, in plot_series
    plot_obj.generate()
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 920, in generate
    self._make_plot()
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1482, in _make_plot
    self._make_ts_plot(data)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1577, in _make_ts_plot
    _plot(data, 0, ax, label, self.style, **kwds)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1553, in _plot
    style=style, **kwds)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tseries/plotting.py", line 82, in tsplot
    left, right = _get_xlim(ax.get_lines())
  File "/home/andrew/git/pandas-rosnfeld/pandas/tseries/plotting.py", line 226, in _get_xlim
    left = min(x[0].ordinal, left)
AttributeError: 'datetime.datetime' object has no attribute 'ordinal'
```

-- ORIGINAL MESSAGE --

Here's a small dataset:

```
date,region,value
1996-12-31,BRA,4.5
2003-12-31,BRA,3.7
2007-12-31,BRA,2.2
1995-12-31,COL,6.3
2000-12-31,COL,4.9
2005-12-31,COL,5.1
2010-12-31,COL,3.4
1997-12-31,PAN,6.3
2003-12-31,PAN,5.1
2008-12-31,PAN,3.9
1990-12-31,VEN,6.7
1991-12-31,VEN,5.4
1992-12-31,VEN,4.5
1993-12-31,VEN,4
1994-12-31,VEN,3.9
1995-12-31,VEN,4.1
1996-12-31,VEN,4.4
1997-12-31,VEN,4.5
1998-12-31,VEN,4.6
1999-12-31,VEN,4.1
2000-12-31,VEN,3.9
2007-12-31,VEN,3.7
```

If I read this in using

``` python
data = pd.read_csv('./data.csv', parse_dates='date', index_col='date')
```

and then try and plot it using

``` python
data.groupby('region').value.plot(legend=True)
```

I get more or less what I expect (perhaps the xlim doesn't go up to 2010-12-31, but otherwise fine).

If I delete out the BRA rows and try this again, I get:

```
Traceback (most recent call last):
  File "repro.py", line 6, in <module>
    data.groupby('region').value.plot()
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 342, in wrapper
    return self.apply(curried)
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 428, in apply
    return self._python_apply_general(f)
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 432, in _python_apply_general
    self.axis)
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 958, in apply
    res = f(group)
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 426, in f
    return func(g, *args, **kwargs)
  File "/home/andrew/git/pandas-rosnfeld/pandas/core/groupby.py", line 333, in curried
    return f(x, *args, **kwargs)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1921, in plot_series
    plot_obj.generate()
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 912, in generate
    self._make_plot()
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1379, in _make_plot
    self._make_ts_plot(data, **self.kwds)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1450, in _make_ts_plot
    _plot(data, 0, ax, label, self.style, **kwds)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tools/plotting.py", line 1434, in _plot
    style=style, **kwds)
  File "/home/andrew/git/pandas-rosnfeld/pandas/tseries/plotting.py", line 82, in tsplot
    left, right = _get_xlim(ax.get_lines())
  File "/home/andrew/git/pandas-rosnfeld/pandas/tseries/plotting.py", line 226, in _get_xlim
    left = min(x[0].ordinal, left)
AttributeError: 'datetime.datetime' object has no attribute 'ordinal'
```

If I delete out both BRA and VEN rows, then there is no exception raised but I only see one series plotted and the x-axis is not formatted as a date.

One could also approach this whole exercise via something like

``` python
data = pd.read_csv('./data.csv', parse_dates='date')
data.pivot('date', 'region', 'value').plot()
```

but this works even worse, I just get a truncated VEN series and nothing else.

This is with current master pandas (but also happens in 0.13.1) and matplotlib 1.3.1.

Are there known issues with plotting sparse-yet-overlapping timeseries?
