Return the last `<c:ser>` element in the last xChart element, based
        on series order (not necessarily the same element as document order).

def last_ser(self):
        """
        Return the last `<c:ser>` element in the last xChart element, based
        on series order (not necessarily the same element as document order).
        """
        last_xChart = self.xCharts[-1]
        sers = last_xChart.sers
        if not sers:
            return None
        return sers[-1]