RRDtool is a popular and versatile program to store and aggregate time-dependent data in way that the size needed for storage is not growing indefinitely. Furthermore it has a powerful plotting engine to generate nice graphs. It is therefore widely used to monitor network devices.
The popular Fritz!Box router can be queried using the TR-064 protocol for the number of transferred bytes. The Fritz Speed package brings these two together to enable monitoring and graphing the WAN traffic of the Fritz!Box.