DataFrame.plot(kind="area") doesn't work with DatetimeIndex
I'm on pandas 0.14.0 / matplotlib 1.3.1.

Given the DataFrame

```
ipdb> print(df)
            Coal    Diesel  Gas     Hydro  Nuclear 
2003-01-31     0  0.893509    0  0.106491        0   
2005-03-31     0  1.218053    0  0.000000        0   
2006-03-31     0  1.218053    0  0.106491        0   
2007-01-31     0  1.218053    0  0.106491        0   
2008-01-31     0  1.218053    0  0.000000        0   
2009-03-31     0  1.218053    0  0.000000        0   
2010-01-31     0  1.218053    0  0.000000        0   
2010-12-31     0  1.218053    0  0.000000        0   
2011-11-30     0  1.218053    0  0.000000        0   
2013-01-31     0  1.218053    0  0.000000        0   

ipdb> df.index
<class 'pandas.tseries.index.DatetimeIndex'>
[2003-01-31, ..., 2013-01-31]
Length: 10, Freq: None, Timezone: None
```

Plotting using `kind='area'` doesn't work:

```
df.plot(ax=ax, grid=False, ylim=(0., 5.),
                                   legend=False, stacked=True,
                                   kind="area")


/home2/hilboll/prj/2014_no2trends-india/code/plots.py in plot_econ_panel_relative(fn_vault, fn_plot)
     81 
     82     axs[3, 0].set_ylabel("Installed capacity (31 Jan 2003 = 1.0)")
---> 83     plt.savefig(fn_plot, bbox_inches="tight")
     84     return states, electr
     85 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/pyplot.pyc in savefig(*args, **kwargs)
    559 def savefig(*args, **kwargs):
    560     fig = gcf()
--> 561     return fig.savefig(*args, **kwargs)
    562 
    563 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/figure.pyc in savefig(self, *args, **kwargs)
   1419             self.set_frameon(frameon)
   1420 
-> 1421         self.canvas.print_figure(*args, **kwargs)
   1422 
   1423         if frameon:

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/backends/backend_qt4agg.pyc in print_figure(self, *args, **kwargs)
    165 
    166     def print_figure(self, *args, **kwargs):
--> 167         FigureCanvasAgg.print_figure(self, *args, **kwargs)
    168         self.draw()

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/backend_bases.pyc in print_figure(self, filename, dpi, facecolor, edgecolor, orientation, format, **kwargs)
   2165                     orientation=orientation,
   2166                     dryrun=True,
-> 2167                     **kwargs)
   2168                 renderer = self.figure._cachedRenderer
   2169                 bbox_inches = self.figure.get_tightbbox(renderer)

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/backend_bases.pyc in print_pdf(self, *args, **kwargs)
   1950         from backends.backend_pdf import FigureCanvasPdf  # lazy import
   1951         pdf = self.switch_backends(FigureCanvasPdf)
-> 1952         return pdf.print_pdf(*args, **kwargs)
   1953 
   1954     def print_pgf(self, *args, **kwargs):

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/backends/backend_pdf.pyc in print_pdf(self, filename, **kwargs)
   2350                                          width, height, image_dpi, RendererPdf(file, image_dpi),
   2351                                          bbox_inches_restore=_bbox_inches_restore)
-> 2352             self.figure.draw(renderer)
   2353             renderer.finalize()
   2354         finally:

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/artist.pyc in draw_wrapper(artist, renderer, *args, **kwargs)
     53     def draw_wrapper(artist, renderer, *args, **kwargs):
     54         before(artist, renderer)
---> 55         draw(artist, renderer, *args, **kwargs)
     56         after(artist, renderer)
     57 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/figure.pyc in draw(self, renderer)
   1032         dsu.sort(key=itemgetter(0))
   1033         for zorder, a, func, args in dsu:
-> 1034             func(*args)
   1035 
   1036         renderer.close_group('figure')

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/artist.pyc in draw_wrapper(artist, renderer, *args, **kwargs)
     53     def draw_wrapper(artist, renderer, *args, **kwargs):
     54         before(artist, renderer)
---> 55         draw(artist, renderer, *args, **kwargs)
     56         after(artist, renderer)
     57 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/axes.pyc in draw(self, renderer, inframe)
   2084 
   2085         for zorder, a in dsu:
-> 2086             a.draw(renderer)
   2087 
   2088         renderer.close_group('axes')

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/artist.pyc in draw_wrapper(artist, renderer, *args, **kwargs)
     53     def draw_wrapper(artist, renderer, *args, **kwargs):
     54         before(artist, renderer)
---> 55         draw(artist, renderer, *args, **kwargs)
     56         after(artist, renderer)
     57 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/axis.pyc in draw(self, renderer, *args, **kwargs)
   1089         renderer.open_group(__name__)
   1090 
-> 1091         ticks_to_draw = self._update_ticks(renderer)
   1092         ticklabelBoxes, ticklabelBoxes2 = self._get_tick_bboxes(ticks_to_draw,
   1093                                                                 renderer)

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/axis.pyc in _update_ticks(self, renderer)
    943 
    944         interval = self.get_view_interval()
--> 945         tick_tups = [t for t in self.iter_ticks()]
    946         if self._smart_bounds:
    947             # handle inverted limits

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/axis.pyc in iter_ticks(self)
    887         Iterate through all of the major and minor ticks.
    888         """
--> 889         majorLocs = self.major.locator()
    890         majorTicks = self.get_major_ticks(len(majorLocs))
    891         self.major.formatter.set_locs(majorLocs)

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/dates.pyc in __call__(self)
    800     def __call__(self):
    801         'Return the locations of the ticks'
--> 802         self.refresh()
    803         return self._locator()
    804 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/dates.pyc in refresh(self)
    817     def refresh(self):
    818         'Refresh internal information based on current limits.'
--> 819         dmin, dmax = self.viewlim_to_dt()
    820         self._locator = self.get_locator(dmin, dmax)
    821 

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/dates.pyc in viewlim_to_dt(self)
    562     def viewlim_to_dt(self):
    563         vmin, vmax = self.axis.get_view_interval()
--> 564         return num2date(vmin, self.tz), num2date(vmax, self.tz)
    565 
    566     def _get_unit(self):

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/dates.pyc in num2date(x, tz)
    309         tz = _get_rc_timezone()
    310     if not cbook.iterable(x):
--> 311         return _from_ordinalf(x, tz)
    312     else:
    313         return [_from_ordinalf(val, tz) for val in x]

/home2/hilboll/prj/2014_no2trends-india/pyenv/lib/python2.7/site-packages/matplotlib/dates.pyc in _from_ordinalf(x, tz)
    212         tz = _get_rc_timezone()
    213     ix = int(x)
--> 214     dt = datetime.datetime.fromordinal(ix)
    215     remainder = float(x) - ix
    216     hour, remainder = divmod(24 * remainder, 1)

ValueError: ordinal must be >= 1
```

The same works just fine when I create the plot replacing `kind='area'` with `kind='bar'`.
