I have three suggestions:

- Logging network traffic on either interface (GPRS/3G and WLAN, i.e. two graphs: one for 3G/GPRS and one for WLAN, both combining RX and TX bytes)
- Logging call duration
- Logging cell signal strength

I posted them with somewhat more info here:
https://garage.maemo.org/tracker/ind...1351&atid=5007

As to how to represent them in the graph... cell signal strength is obvious, a line. Call duration and network traffic is somewhat similar, could be a line or bar to show bytes/minutes per interval/period (fixed/user configured/dynamic according to zoom level), or (esp. for call duration) it could be a line showing accumulated call time since the previous (full) charge, as this is a value commonly advertised.