Add `c:ser` elements to the last xChart element in *plotArea*, cloned
        from the last `c:ser` child of that last xChart.

def _add_cloned_sers(self, plotArea, count):
        """
        Add `c:ser` elements to the last xChart element in *plotArea*, cloned
        from the last `c:ser` child of that last xChart.
        """
        def clone_ser(ser):
            new_ser = deepcopy(ser)
            new_ser.idx.val = plotArea.next_idx
            new_ser.order.val = plotArea.next_order
            ser.addnext(new_ser)
            return new_ser

        last_ser = plotArea.last_ser
        for _ in range(count):
            last_ser = clone_ser(last_ser)