Compare Feature

It would be super powerful to be able to easily compare historical data quickly. the two examples I can think of:

Data Source X - Today v. Last Monday

Web server 1 CPU v. Webserver 2 CPU

I would view this as an investigative tool that allows you to quickly analyze if something looks out of sorts... being able to put last Monday and this Monday on the same graph allows for a very easy comparison. This would be used a lot in the heat of the moment or for post change validation of performance.