for some of services when i click view trends for this service an empty graph is shown, why ? :?
Try changing the report period to "this month" and see if that helps and/or change the first assumed state to "ok".
changing the report period to this month or other period didn't help,but when i changed the First assumed state to current state or some others state the graph appeared.but in the case of some servers this option is UnSpecified and the graph is shown,why is so ?
To fully understand how those graphs work, takes a bit of inner knowledge of the workings of nagios. The developer would only know that. Bottom line is, make changes like you did, if you don't see what you want to see. That's all I do.