each_period=cohorts['FirstOrder'].value_counts().sort_index() each_period
2009-01 15 2009-02 14 2009-03 13 2009-04 12 2009-05 11 2009-06 10 2009-07 9 2009-08 8 2009-09 7 2009-10 6 2009-11 5 2009-12 4 2010-01 3 2010-02 2 2010-03 1 Name: FirstOrder, dtype: int64