TypeError when plotting TimeSeries with NaN data, maybe a BUG?
I get a TypeError message when I try to plot a pandas TimeSeries object that contains NaN's. The strange thing is, this does not happen when I have 'inf' values, or when a normal series is used with integer index values. 

Now, all of a sudden (no recent updates I can think of) all my plotting scripts get the TypeError, but before they worked fine. I can do dropna() before, but then missing data is interpolated in the plot, which I do not want; the data is after all missing...  If I do fillna(np.inf), things are plotting as I like. 

As discussed below, I am not sure this is a bug in pandas 0.12 / matplotlib 1.3 but something can be wrong with my installation. I just don't know what that can be and a fresh install is not an option now. I hope someone can test the code and provide ideas on how I can solve the problem?

The following code causes problems:

``` python
import numpy as np
import pandas as pd
s = pd.Series(np.arange(10.))
rng = pd.date_range('2012-12-12', periods=10, freq='H')
ts = pd.Series(np.arange(10.), index=rng)

ts.plot()  # Works fine
s.plot()   # Works fine
# now, if I replace one value with an NaN
ts[2] = np.NaN

ts.plot() # gives the following error:

Traceback (most recent call last):

  File "<ipython-input-6-132f3667ee95>", line 1, in <module>
    ts.plot()

  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1730, in plot_series
    plot_obj.generate()

  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 856, in generate
    self._make_plot()

  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1240, in _make_plot
    self._make_ts_plot(data, **self.kwds)

  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1311, in _make_ts_plot
    _plot(data, 0, ax, label, self.style, **kwds)

  File "C:\Python27\lib\site-packages\pandas\tools\plotting.py", line 1295, in _plot
    style=style, **kwds)

  File "C:\Python27\lib\site-packages\pandas\tseries\plotting.py", line 77, in tsplot
    lines = plotf(ax, *args, **kwargs)

  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 4137, in plot
    for line in self._get_lines(*args, **kwargs):

  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 317, in _grab_next_args
    for seg in self._plot_args(remaining, kwargs):

  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 295, in _plot_args
    x, y = self._xy_from_xy(x, y)

  File "C:\Python27\lib\site-packages\matplotlib\axes.py", line 214, in _xy_from_xy
    by = self.axes.yaxis.update_units(y)

  File "C:\Python27\lib\site-packages\matplotlib\axis.py", line 1336, in update_units
    converter = munits.registry.get_converter(data)

  File "C:\Python27\lib\site-packages\matplotlib\units.py", line 137, in get_converter
    xravel = x.ravel()

  File "C:\Python27\lib\site-packages\numpy\ma\core.py", line 3969, in ravel
    r._mask = ndarray.ravel(self._mask).reshape(r.shape)

  File "C:\Python27\lib\site-packages\pandas\core\series.py", line 981, in reshape
    return ndarray.reshape(self, newshape, order)

TypeError: an integer is required

# If I do:
 ts[2] = np.inf   # Then ts.plot() works...
```

I am using pandas 0.12 with matplotlib 1.3 on python x,y 2.7.5.1 on Windows 7.
Upgrading to matplotlib 1.3.1 or downgrading to pandas 0.11 did not solve the problem.

I have tried the same code in an WinPython-64bit -2.7.4.1.amd64 that I have installed and then the code works fine. Dependencies are then:
pandas 0.11 and matplotlib 1.2.1. Maybe this indicates something is wrong with my python x,y installation?

Happy to provide more details. Looking forward to your help!

Maarten
