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

        Key values are between 1-365.