def write_ser(series) # :nodoc:
      @writer.tag_elements('c:ser') do
        write_ser_base(series) do
          write_c_invert_if_negative(series.invert_if_negative)
        end
        # Write the c:cat element.
        write_cat(series)
        # Write the c:val element.
        write_val(series)
        # Write the c:smooth element.
        write_c_smooth(series.smooth) if ptrue?(@smooth_allowed)
      end
      @series_index += 1
    end
def write_ser(series) # :nodoc:
      @writer.tag_elements('c:ser') do
        write_ser_base(series) do
          write_c_invert_if_negative(series.invert_if_negative)
        end
        # Write the c:cat element.
        write_cat(series)
        # Write the c:val element.
        write_val(series)
        # Write the c:smooth element.
        write_c_smooth(series.smooth) if ptrue?(@smooth_allowed)
      end
      @series_index += 1
    end
Write the <c:ser> element.