As has been pointed out, Tomato already does pretty graphs. DD-WRT v24 also has a traffic monitor that shows graphs of traffic in/out on the LAN, WAN, and wireless on individual graphs. You can also add BW logging to DD-WRT:
http://www.dd-wrt.com/wiki/index.php/BWlog    http://www.hetos.de/bwlog.html

However, for proper monitoring, the router should generate and collect SNMP data. The data collection programs and graphing are best done by a dedicated management PC. I use MRTG, PRTG, RRDTool, and various other SNMP apps for data collection and graphing.
http://www.dd-wrt.com/wiki/index.php/Multi_Router_Traffic_Grapher
I've also installed the MRTG data collector in the router, but that requires more RAM than most routers have available or installing an SD memory card which requires soldering and butchery.

The big problem with monitoring is that many of the wireless chip level numbers are not easily available. Important items such as the wireless error and retransmission rates, are not available. The supplied OID's are adequate for layer 3 (IP) monitoring, but not layer 2 (MAC).

Incidentally, before suggesting any more features, you might want to look at the DD-WRT tutorials section, which details some of the more obscure and exotic features.
http://www.dd-wrt.com/wiki/index.php/Tutorials

Tomato for WGR614L

Right here

http://www.myopenrouter.com/download/10164/Tomato-for-WGR614L-20080204v11/

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