I'm looking for a tool to display data from performance monitor counters. I'm looking for something that would be visually appealing (look like a dashboard) and it should be able to aggregate (sum up) over multiple counters. No thresholds/alarming needed, we are using Nagios for that.
I would look into Cacti for this. It is fairly customizable, with respect to exactly what you are graphing, and how the data should be interpreted. It should be capable of doing what you need.
I'll throw my vote in for Circonus (http://circonus.com/). You can do all of that, including derived values, stacked graphs and correlate virtually any metrics. And if you decide later on to move away from Nagios, it also allows you to set rules on your metrics for alerting.