def list_processed_parameter_group_histogram(self, group=None, start=None, stop=None, merge_time=20):
        """
        Reads index records related to processed parameter groups between the
        specified start and stop time.

        Each iteration returns a chunk of chronologically-sorted records.

        :param float merge_time: Maximum gap in seconds before two consecutive index records are merged together.
        :rtype: ~collections.Iterable[.IndexGroup]
        """
        params = {}
        if group is not None:
            params['group'] = group
        if start is not None:
            params['start'] = to_isostring(start)
        if stop is not None:
            params['stop'] = to_isostring(stop)
        if merge_time is not None:
            params['mergeTime'] = int(merge_time * 1000)

        return pagination.Iterator(
            client=self._client,
            path='/archive/{}/parameter-index'.format(self._instance),
            params=params,
            response_class=archive_pb2.IndexResponse,
            items_key='group',
            item_mapper=IndexGroup,
        )