def group_by_month(self):
data_by_month = OrderedDict()<EOL>for d in xrange(<NUM_LIT:1>, <NUM_LIT>):<EOL><INDENT>data_by_month[d] = []<EOL><DEDENT>for v, dt in zip(self._values, self.datetimes):<EOL><INDENT>data_by_month[dt.month].append(v)<EOL><DEDENT>return data_by_month<EOL>
Return a dictionary of this collection's values grouped by each month.

        Key values are between 1-12.