def to_xml_string(str = '')
      # macro attribute should be optional!
      str << '<xdr:graphicFrame>'
      str << '<xdr:nvGraphicFramePr>'
      str << ('<xdr:cNvPr id="' << @anchor.drawing.index.to_s << '" name="' << 'item_' << @anchor.drawing.index.to_s << '"/>')
      str << '<xdr:cNvGraphicFramePr/>'
      str << '</xdr:nvGraphicFramePr>'
      str << '<xdr:xfrm>'
      str << '<a:off x="0" y="0"/>'
      str << '<a:ext cx="0" cy="0"/>'
      str << '</xdr:xfrm>'
      str << '<a:graphic>'
      str << ('<a:graphicData uri="' << XML_NS_C << '">')
      str << ('<c:chart xmlns:c="' << XML_NS_C << '" xmlns:r="' << XML_NS_R << '" r:id="' << rId << '"/>')
      str << '</a:graphicData>'
      str << '</a:graphic>'
      str << '</xdr:graphicFrame>'
    end
def to_xml_string(str = '')
      # macro attribute should be optional!
      str << '<xdr:graphicFrame>'
      str << '<xdr:nvGraphicFramePr>'
      str << ('<xdr:cNvPr id="' << @anchor.drawing.index.to_s << '" name="' << 'item_' << @anchor.drawing.index.to_s << '"/>')
      str << '<xdr:cNvGraphicFramePr/>'
      str << '</xdr:nvGraphicFramePr>'
      str << '<xdr:xfrm>'
      str << '<a:off x="0" y="0"/>'
      str << '<a:ext cx="0" cy="0"/>'
      str << '</xdr:xfrm>'
      str << '<a:graphic>'
      str << ('<a:graphicData uri="' << XML_NS_C << '">')
      str << ('<c:chart xmlns:c="' << XML_NS_C << '" xmlns:r="' << XML_NS_R << '" r:id="' << rId << '"/>')
      str << '</a:graphicData>'
      str << '</a:graphic>'
      str << '</xdr:graphicFrame>'
    end
Serializes the object
 @param [String] str
 @return [String]