def group_by_month(self):
        """Return a dictionary of this collection's values grouped by each month.

        Key values are between 1-12.
        """
        data_by_month = OrderedDict()
        for d in xrange(1, 13):
            data_by_month[d] = []
        for v, dt in zip(self._values, self.datetimes):
            data_by_month[dt.month].append(v)
        return data_by_month
Return a dictionary of this collection's values grouped by each month.

        Key values are between 1-12.