Business frequencies error in tsplot
Should alias these to their calendar frequencies for compatibility, or something

```
/home/wesm/code/pandas/pandas/tseries/plotting.py in tsplot(series, plotf, *args, **kwargs)
     77     # Convert DatetimeIndex to PeriodIndex
     78     if isinstance(series.index, DatetimeIndex):
---> 79         idx = series.index.to_period(freq=freq)
     80         series = Series(series.values, idx, name=series.name)
     81 

/home/wesm/code/pandas/pandas/tseries/index.pyc in to_period(self, freq)
    528             freq = self.freq
    529 
--> 530         return PeriodIndex(self.values, freq=freq)
    531 
    532     def order(self, return_indexer=False, ascending=True):

/home/wesm/code/pandas/pandas/tseries/period.pyc in __new__(cls, data, freq, start, end, periods, copy, name)
    516             freq = freq.freq
    517         else:
--> 518             freq = _freq_mod.get_standard_freq(freq)
    519 
    520         if data is None:

/home/wesm/code/pandas/pandas/tseries/frequencies.pyc in get_standard_freq(freq)
    454         return get_offset_name(freq)
    455 
--> 456     code, stride = get_freq_code(freq)
    457     return _get_freq_str(code, stride)
    458 

/home/wesm/code/pandas/pandas/tseries/frequencies.pyc in get_freq_code(freqstr)
     64 
     65     base, stride = _base_and_stride(freqstr)
---> 66     code = _period_str_to_code(base)
     67 
     68     return code, stride

/home/wesm/code/pandas/pandas/tseries/frequencies.pyc in _period_str_to_code(freqstr)
    646         return _period_code_map[freqstr]
    647     except:
--> 648         alias = _period_alias_dict[freqstr]
    649         try:
    650             return _period_code_map[alias]

KeyError: 'BM'
```
