'Cannot compare tz-naive and tz-aware timestamps' when plotting series
Pandas version 0.10.1:

```
import pandas as pd
index = pd.date_range('1/1/2011', periods=2, freq='H', tz='Europe/Brussels')
ts = pd.Series([188.5, 328.25], index=index)
ts.plot()
```

yields the following exception:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1534, in plot_series
    plot_obj.generate()
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 735, in generate
    self._make_plot()
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1076, in _make_plot
    if not self.x_compat and self.use_index and self._use_dynamic_x():
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1057, in _use_dynamic_x
    return (freq is not None) and self._is_dynamic_freq(freq)
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1030, in _is_dynamic_freq
    return freq is not None and self._no_base(freq)
  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1043, in _no_base
    return Period(x[0], freq).to_timestamp() == x[0]
  File "tslib.pyx", line 434, in pandas.tslib._Timestamp.__richcmp__ (pandas\tslib.c:7930)
  File "tslib.pyx", line 484, in pandas.tslib._Timestamp._assert_tzawareness_compat (pandas\tslib.c:8528)
Exception: Cannot compare tz-naive and tz-aware timestamps
```
