def to_xml_string(str = '')
      validate_attributes_for_chart_type
      str << '<c:dLbls>'
      %w(d_lbl_pos show_legend_key show_val show_cat_name show_ser_name show_percent show_bubble_size show_leader_lines).each do |key|
        next unless instance_values.keys.include?(key) && instance_values[key] != nil
        str <<  "<c:#{Axlsx::camel(key, false)} val='#{instance_values[key]}' />" 
      end
      str << '</c:dLbls>'
    end
serializes the data labels
@return [String]