Adjust the number of c:ser elements in *plotArea* to *new_ser_count*.
        Excess c:ser elements are deleted from the end, along with any xChart
        elements that are left empty as a result. Series elements are
        considered in xChart + series order. Any new c:ser elements required
        are added to the last xChart element and cloned from the last c:ser
        element in that xChart.

def _adjust_ser_count(self, plotArea, new_ser_count):
        """
        Adjust the number of c:ser elements in *plotArea* to *new_ser_count*.
        Excess c:ser elements are deleted from the end, along with any xChart
        elements that are left empty as a result. Series elements are
        considered in xChart + series order. Any new c:ser elements required
        are added to the last xChart element and cloned from the last c:ser
        element in that xChart.
        """
        ser_count_diff = new_ser_count - len(plotArea.sers)
        if ser_count_diff > 0:
            self._add_cloned_sers(plotArea, ser_count_diff)
        elif ser_count_diff < 0:
            self._trim_ser_count_by(plotArea, abs(ser_count_diff))