So nagios can be good at graphing particular resource utilisation or other metrics, but I'm looking for a tool that can output a chart or other graphical representation of how much CPU time/CPU utilisation /all/ services on a server are currently consuming. I think New Relic could probably achieve this to an extent, but I was wondering if there was a popular open source app used for this.

In case I am explaining this in a bad way, my actual problem is that I have a shared server with suexec enabled (ie. httpd cgi running under multiple user accounts). I'd like to know which users are using the most CPU during periods of the day.