Rrdtool is a program, that allows you to measure counters from all kinds of datasources, and put them into Round Robin Databases. From that database you can make graphs. In this way you are able to measure dataflows on your network, the CPU load ... whatever. You just have to write the scripts to get the data.
Rrdtool can be downloaded from Tobi Oetiker's site and is currently at version 1.0.48
After configuring with ./configure -build=i586-pc-linux-gnu i typed make to build it.
The build produced a 940 KB executable, after stripping only 315 KB remains.
I copy this executable to the /bin directory in the image. And now it's up to you to make some scripts.
Now, making all the graphs doen't make sense, so the next thing you will need is a webserver to make them accessable.