def to_xml_string(str = '', options = {})
      if options[:ids]
        # CatAxis must come first in the XML (for Microsoft Excel at least)
        sorted = axes.sort_by { |name, axis| axis.kind_of?(CatAxis) ? 0 : 1 }
        sorted.each { |axis| str << ('<c:axId val="' << axis[1].id.to_s << '"/>') }        
      else
        axes.each { |axis| axis[1].to_xml_string(str) }
      end
    end
Serializes the object
@param [String] str
@param [Hash] options
@option options ids
If the ids option is specified only the axis identifier is
serialized. Otherwise, each axis is serialized in full.