Series.plot broken for Int64Index
Sorry to do my daily issue bombardment, but I live on the edge for better or worse. I thought this should work. It does if it's a single column DataFrame ie., `y.to_frame().plot()`

```
[~/]
[121]: y = pd.util.testing.makeStringSeries()

[~/]
[122]: y.reset_index(drop=True, inplace=True)

[~/]
[123]: y.plot()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-123-2d08882c171f> in <module>()
----> 1 y.plot()

/home/skipper/src/pandas-skipper/pandas/tools/plotting.pyc in plot_series(series, label, kind, use_index, rot, xticks, yticks, xlim, ylim, ax, style, grid, legend, logx, logy, secondary_y, **kwds)
1919                      secondary_y=secondary_y, **kwds)
1920 
-> 1921     plot_obj.generate()
1922     plot_obj.draw()
1923 

/home/skipper/src/pandas-skipper/pandas/tools/plotting.pyc in generate(self)
    910         self._compute_plot_data()
    911         self._setup_subplots()
--> 912         self._make_plot()
    913         self._post_plot_logic()
    914         self._adorn_subplots()

/home/skipper/src/pandas-skipper/pandas/tools/plotting.pyc in _make_plot(self)
1377         if not self.x_compat and self.use_index and self._use_dynamic_x():
1378             data = self._maybe_convert_index(self.data)
-> 1379             self._make_ts_plot(data, **self.kwds)
1380         else:
1381             lines = []

/home/skipper/src/pandas-skipper/pandas/tools/plotting.pyc in _make_ts_plot(self, data, **kwargs)
1448             self._maybe_add_color(colors, kwds, style, 0)
1449 
-> 1450             _plot(data, 0, ax, label, self.style, **kwds)
1451         else:
1452             for i, col in enumerate(data.columns):

/home/skipper/src/pandas-skipper/pandas/tools/plotting.pyc in _plot(data, col_num, ax, label, style, **kwds)
1432         def _plot(data, col_num, ax, label, style, **kwds):
1433             newlines = tsplot(data, plotf, ax=ax, label=label,
-> 1434                                 style=style, **kwds)
1435             ax.grid(self.grid)
1436             lines.append(newlines[0])

/home/skipper/src/pandas-skipper/pandas/tseries/plotting.pyc in tsplot(series, plotf, **kwargs)
    80     # set date formatter, locators and rescale limits
    81     format_dateaxis(ax, ax.freq)
---> 82     left, right = _get_xlim(ax.get_lines())
    83     ax.set_xlim(left, right)
    84 

/home/skipper/src/pandas-skipper/pandas/tseries/plotting.pyc in _get_xlim(lines)
    224     for l in lines:
    225         x = l.get_xdata()
--> 226         left = min(x[0].ordinal, left)
    227         right = max(x[-1].ordinal, right)
    228     return left, right

AttributeError: 'numpy.int64' object has no attribute 'ordinal'

[~/]
[124]: pd.version.version
[124]: '0.13.1-426-ge19b2eb'
```
