Possible bug: No numeric data error when numeric data added
New to pandas, so not sure if bug or stupidity, but the behavior is not what I expect. Pandas version is 0.12.0 on Mac OS X with python 2.7.5. 

In [12]: df = pd.DataFrame({'a': [3, 3], 'b': [3, 3]})

In [13]: df.plot()
Out[13]: <matplotlib.axes.AxesSubplot at 0x10fb6e2d0>

In [14]: df_two = pd.DataFrame({'a': [None, None], 'b': [None, None]})

In [15]: df_two
Out[15]: 
      a     b
0  None  None
1  None  None

In [16]: df_two = df_two.fillna(3)

In [17]: df_two
Out[17]: 
   a  b
0  3  3
1  3  3
## In [18]: df_two.plot()

TypeError                                 Traceback (most recent call last)
<ipython-input-18-7a340ea12677> in <module>()
----> 1 df_two.plot()

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/tools/plotting.pyc in plot_frame(frame, x, y, subplots, sharex, sharey, use_index, figsize, grid, legend, rot, ax, style, title, xlim, ylim, logx, logy, xticks, yticks, kind, sort_columns, fontsize, secondary_y, *_kwds)
   1634                      logy=logy, sort_columns=sort_columns,
   1635                      secondary_y=secondary_y, *_kwds)
-> 1636     plot_obj.generate()
   1637     plot_obj.draw()
   1638     if subplots:

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/tools/plotting.pyc in generate(self)
    852     def generate(self):
    853         self._args_adjust()
--> 854         self._compute_plot_data()
    855         self._setup_subplots()
    856         self._make_plot()

/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/tools/plotting.pyc in _compute_plot_data(self)
    947         if is_empty:
    948             raise TypeError('Empty {0!r}: no numeric data to '
--> 949                             'plot'.format(numeric_data.__class__.**name**))
    950 
    951         self.data = numeric_data

TypeError: Empty 'DataFrame': no numeric data to plot
