Get the single valued aggregations with respect to the
        previous time interval.

def aggregations(self):
        """Get the single valued aggregations with respect to the
        previous time interval."""

        prev_month_start = get_prev_month(self.end, self.query.interval_)
        self.query.since(prev_month_start)
        agg = super().aggregations()
        if agg is None:
            agg = 0  # None is because NaN in ES. Let's convert to 0
        return agg