Hi
We're getting into Analysis best practices here :-)
If you want to do this in Controller, you can overlay the Vuser graphs on top of the TRT graph; that can give you an idea in real time about the affect of Vuser numbers on your AUT.
The better approach (in my opinion), is to do this after the fact using Analysis.
press on the "Analyze Results" button in the Controller, it will open the Analysis with the data already there.
once in Analysis, you can group the average TRT graph by Vuser id and you can get the exact data you need per Vuser.
That being said, TRT is not the only graph to monitor when trying to asses the cause of system slowing down. Always monitor system resources as well and from there you can achieve more data as to what are the limits of your system.
Good Luck,
Lior