You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: series.rb
    path of file: ./repos/caxlsx/lib/axlsx/drawing
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module Axlsx
  # A Series defines the common series attributes and is the super class for all concrete series types.
  # @note The recommend
    
    def to_xml_string(str = +'')
      str << '<c:ser>'
      str << '<c:idx val="' << index.to_s << '"/>'
      str << '<c:order val="' << (order || index).to_s << '"/>'
      title.to_xml_string(str) unless title.nil?
      yield if block_given?
      str << '</c:ser>'
    end