plot() fails with a combined bar and line graph
When I plot one column of a df as a bar, run twinx(), and plot another as a line, I get the following error:
(using pandas 0.7.0 and numpy 1.5.1)

In [94]: Traceback (most recent call last):
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, _args, *_kwargs)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/figure.py", line 886, in draw
    func(_args)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, *args, *_kwargs)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/axes.py", line 1983, in draw
    a.draw(renderer)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/artist.py", line 55, in draw_wrapper
    draw(artist, renderer, _args, *_kwargs)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/axis.py", line 1041, in draw
    ticks_to_draw = self._update_ticks(renderer)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/axis.py", line 931, in _update_ticks
    tick_tups = [ t for t in self.iter_ticks()]
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/axis.py", line 878, in iter_ticks
    majorLocs = self.major.locator()
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/dates.py", line 752, in __call__
    self.refresh()
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/dates.py", line 761, in refresh
    dmin, dmax = self.viewlim_to_dt()
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/dates.py", line 533, in viewlim_to_dt
    return num2date(vmin, self.tz), num2date(vmax, self.tz)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/dates.py", line 292, in num2date
    if not cbook.iterable(x): return _from_ordinalf(x, tz)
  File "/intucell/IntuRMS/intustall/deps/osx/lib/python2.7/site-packages/matplotlib/dates.py", line 206, in _from_ordinalf
    dt = datetime.datetime.fromordinal(ix)
ValueError: ordinal must be >= 1

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