Comment 59 for bug 93847

On my side, the "update interval" is correct, the graph respects the scale on the lowest bar.

But about CPU usage, I've got the same behaviour.
Before launching Gnome-System-Monitor, X uses less than 1% of CPU.

When I run gnome-system-monitor, X and g-s-m are using each 8 to 12% of CPU. So the CPU overhead is between 15 and 23%.
Switch the "update interval" to 15'' decreases the CPU usage to 1 - 2% for the each, but the graph is dramaticaly slowed down.
When I go to the "Process" tab, the CPU usage grows fast to above 50% after reverting back to 10% of CPU. It is the same when I launch gnome-system-monitor.
This is very annoying if you want to kill a process that already consumes to much CPU !

I also use the system monitor applet, and is showing the same graphs (and even more) and it is using less than 1% of CPU.

All CPU usage are for the all CPU capacity of my i5-750. That means 100% of CPU usage shows that all 4 cores are used at 100%.