code: def _add_cloned_sers(self, plotArea, count):
          
          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)
docs: Add `c:ser` elements to the last xChart element in *plotArea*, cloned
          from the last `c:ser` child of that last xChart.