def to_xml_string(idx, str = "")
      Axlsx::validate_unsigned_int(idx)
      if !v.to_s.empty?
        str << ('<c:pt idx="' << idx.to_s << '"><c:v>' << ::CGI.escapeHTML(v.to_s) << '</c:v></c:pt>')
      end
    end
def to_xml_string(idx, str = "")
      Axlsx::validate_unsigned_int(idx)
      if !v.to_s.empty?
        str << ('<c:pt idx="' << idx.to_s << '"><c:v>' << ::CGI.escapeHTML(v.to_s) << '</c:v></c:pt>')
      end
    end
serialize the object