Plotting broken if inferred_freq has a multiple
Also reported on mailing list

```
In [4]: ts.plot()
Out[4]: <matplotlib.axes.AxesSubplot at 0x41cc310>

In [5]: ts.index.fr
ts.index.freq     ts.index.freqstr  

In [5]: ts.index.freq
Out[5]: <10 Days>

In [6]: ts.index.freq = None
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/<ipython-input-6-d89557d7f8e5> in <module>()
----> 1 ts.index.freq = None

AttributeError: can't set attribute

In [7]: ts.index.offset = None

In [8]: ts
Out[8]: 
2000-01-01    2.963378
2000-01-11   -0.685998
2000-01-21    0.392812
2000-01-31   -0.143893
2000-02-10   -1.162283
2000-02-20   -0.460511
2000-03-01   -0.751741
2000-03-11   -0.567428
2000-03-21   -0.439105
2000-03-31    0.040322
2000-04-10   -0.929786
2000-04-20   -0.620005
2000-04-30    0.482502
2000-05-10    0.109013
2000-05-20    0.657817
2000-05-30   -0.668386
2000-06-09   -1.797074
2000-06-19   -0.197738
2000-06-29   -0.312481
2000-07-09    1.240477
2000-07-19    0.447583
2000-07-29    0.188429
2000-08-08   -0.117540
2000-08-18   -1.145495
2000-08-28    0.479124
2000-09-07    0.309968
2000-09-17    0.769928
2000-09-27   -0.501629
2000-10-07    0.164322
2000-10-17   -0.554113
2000-10-27    0.468588
2000-11-06    0.857458
2000-11-16    0.508256
2000-11-26   -0.452384
2000-12-06    1.732538
2000-12-16    0.695495
2000-12-26    0.441233
2001-01-05   -0.271561

In [9]: ts.index.offset = None

In [10]: ts.plot()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/<ipython-input-10-132f3667ee95> in <module>()
----> 1 ts.plot()

/home/wesm/code/pandas/pandas/tools/plotting.pyc in plot_series(series, label, kind, use_index, rot, xticks, yticks, xlim, ylim, ax, style, grid, logy, **kwds)
    758                      legend=False, grid=grid, label=label, **kwds)
    759 
--> 760     plot_obj.generate()
    761     plot_obj.draw()
    762 

/home/wesm/code/pandas/pandas/tools/plotting.pyc in generate(self)
    239         self._compute_plot_data()
    240         self._setup_subplots()
--> 241         self._make_plot()
    242         self._post_plot_logic()
    243         self._adorn_subplots()

/home/wesm/code/pandas/pandas/tools/plotting.pyc in _make_plot(self)
    426         if self.use_index and self.has_ts_index:
    427             data = self._maybe_convert_index(self.data)
--> 428             self._make_ts_plot(data)
    429         else:
    430             x = self._get_xticks()

/home/wesm/code/pandas/pandas/tools/plotting.pyc in _make_ts_plot(self, data, **kwargs)
    478 
    479             label = com._stringify(self.label)
--> 480             tsplot(data, plotf, ax=ax, label=label, **kwargs)
    481             ax.grid(self.grid)
    482         else:

/home/wesm/code/pandas/pandas/tseries/plotting.pyc in tsplot(series, plotf, *args, **kwargs)
    114     # format args and lot
    115     args = _check_plot_params(series, series.index, freq, *args)
--> 116     plotted = plotf(ax, *args,  **kwargs)
    117 
    118     format_dateaxis(ax, ax.freq)

/home/wesm/code/repos/matplotlib/lib/matplotlib/axes.pyc in plot(self, *args, **kwargs)
   3892         lines = []
   3893 
-> 3894         for line in self._get_lines(*args, **kwargs):
   3895             self.add_line(line)
   3896             lines.append(line)

/home/wesm/code/repos/matplotlib/lib/matplotlib/axes.pyc in _grab_next_args(self, *args, **kwargs)
    321                 return
    322             if len(remaining) <= 3:
--> 323                 for seg in self._plot_args(remaining, kwargs):
    324                     yield seg
    325                 return

/home/wesm/code/repos/matplotlib/lib/matplotlib/axes.pyc in _plot_args(self, tup, kwargs)
    299             x = np.arange(y.shape[0], dtype=float)
    300 
--> 301         x, y = self._xy_from_xy(x, y)
    302 
    303         if self.command == 'plot':

/home/wesm/code/repos/matplotlib/lib/matplotlib/axes.pyc in _xy_from_xy(self, x, y)
    215     def _xy_from_xy(self, x, y):
    216         if self.axes.xaxis is not None and self.axes.yaxis is not None:
--> 217             bx = self.axes.xaxis.update_units(x)
    218             by = self.axes.yaxis.update_units(y)
    219 

/home/wesm/code/repos/matplotlib/lib/matplotlib/axis.pyc in update_units(self, data)
   1276         """
   1277 
-> 1278         converter = munits.registry.get_converter(data)
   1279         if converter is None:
   1280             return False

/home/wesm/code/repos/matplotlib/lib/matplotlib/units.pyc in get_converter(self, x)
    131 
    132         if converter is None and iterable(x):
--> 133             for thisx in x:
    134                 # Make sure that recursing might actually lead to a solution, if
    135                 # we are just going to re-examine another item of the same kind,

/home/wesm/code/pandas/pandas/tseries/period.pyc in __iter__(self)
    685     def __iter__(self):
    686         for val in self.values:
--> 687             yield Period(ordinal=val, freq=self.freq)
    688 
    689     @property

/home/wesm/code/pandas/pandas/tseries/period.pyc in __init__(self, value, freq, ordinal, year, month, quarter, day, hour, minute, second)
    136         base, mult = _gfc(freq)
    137         if mult != 1:
--> 138             raise ValueError('Only mult == 1 supported')
    139 
    140         if self.ordinal is None:

ValueError: Only mult == 1 supported
```
