BUG: Series.plot() doesn't work with CustomBusinessDay frequency
Closes #7222
Missing freqstr 'C' in `_period_code_map`
I found `pandas/tseries/tests/test_frequencies.py:TestFrequencyCode.test_freq_code` cover this change.

tests!
 `pandas/tseries/tests/test_frequencies.py:TestFrequencyCode.test_freq_code` cover `_period_code_map` change itself.
however, I think it's also good idea to add a test on `test_graphics.py` since the original issue was related on plotting.

be back soon.
 added a test.
 thanks @scari 
