How to define clear in nagiosgraph.conf?


I met a show.cgi problem ,I got the rrd files but when I open show.cgi page .It doesnt work properly. I check the nagiosgraph.log. i found these description:“Sat Sep 19 07:40:00 2009 show.cgi warn - define ‘clear’ in nagiosgraph.conf”.who can tell me how to define it in nagiosgraph?
Thanks in advance