def group_by_day(self):
        """Return a dictionary of this collection's values grouped by each day of year.

        Key values are between 1-365.
        """
        hourly_data_by_day = OrderedDict()
        for d in xrange(1, 366):
            hourly_data_by_day[d] = []
        a_per = self.header.analysis_period
        indx_per_day = 24 * a_per.timestep
        start_doy = sum(a_per._num_of_days_each_month[:a_per.st_time.month-1]) \
            + a_per.st_time.day
        if not a_per.is_reversed:
            for i in range(0, len(self._values), indx_per_day):
                hourly_data_by_day[start_doy] = self._values[i:i + indx_per_day]
                start_doy += 1
        else:
            end_ind = 24 * a_per.timestep * (365 - start_doy)
            for i in range(0, end_ind + 1, indx_per_day):
                hourly_data_by_day[start_doy] = self._values[i:i + indx_per_day]
                start_doy += 1
            start_doy = 1
            for i in range(end_ind, len(self._values), indx_per_day):
                hourly_data_by_day[start_doy] = self._values[i:i + indx_per_day]
                start_doy += 1
        return hourly_data_by_day
Return a dictionary of this collection's values grouped by each day of year.

        Key values are between 1-365.