Plotting Series/DF of dtype object fails intermittently
There seems to be an intermittent issue which prevents plotting Series/DataFrames with dtype `object`. See traceback below. This manifests for me when reading data from files but not when manually creating dataframes from the command line. It can be worked around by explicitly casting to float64. The presence of NANs does not appear to have an effect.

``` python
>>> df = pd.read_csv('data.csv')  # CSV data file with some "NAN" entries
>>> df['chan1'].dtype
dtype('object')

>>> df['chan1'].plot()  # fails
>>> df['chan1'].dropna().plot()  # fails
>>> df['chan1'].apply(float).plot()  # works
>>> df['chan1'].dropna().apply(float).plot()  # works
```

This works though:

``` python
df = pd.DataFrame([3, 1, 4, np.nan, 5, 9], dtype=object)  # works
df = pd.DataFrame([3, 1, 4, "NAN", 5, 9], dtype=object)  # works
df = pd.DataFrame([3, 1, 4, 1, 5, 9], dtype=object)  # works
```

Here is the traceback; it is the same as encountered in #478 and presumably in #1818. Note it doesn't include the error message added in 9186cbc

``` python
>>> df = pd.read_csv(r'C:\test\stats.dat')
>>> df['Ts_Avg'].plot()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\pandas\tools\plotting.py", line 1534, in plot_series
    plot_obj.generate()
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\pandas\tools\plotting.py", line 735, in generate
    self._make_plot()
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\pandas\tools\plotting.py", line 1106, in _make_plot
    newline = plotf(*args, **kwds)[0]
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\matplotlib\axes.py", line 3996, in plot
    for line in self._get_lines(*args, **kwargs):
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\matplotlib\axes.py", line 330, in _grab_next_args
    for seg in self._plot_args(remaining, kwargs):
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\matplotlib\axes.py", line 289, in _plot_args
    linestyle, marker, color = _process_plot_format(tup[-1])
  File "C:\WinPython-64bit-2.7.3.3\python-2.7.3.amd64\lib\site-packages\matplotlib\axes.py", line 96, in _process_plot_format
    if fmt.find('--')>=0:
AttributeError: 'numpy.ndarray' object has no attribute 'find'
```
