Mrtg is very versatile and can log just about anything graphically. There is a great document on this at: http://www.willy.com/Scripps/mrtg-data.html.
In less than two pages, Willy describes how to log other data using mrtg. I will leverage that to document how I remotely log the signal strength and quality of my Cisco Aironet ISA-342 card.
Snmpd has the ability to execute a command on the target system (in our case, the Dachstein firewall) and return the output (and other infomation) from the script. We will use this feature to run two short and simple scripts, checksig1 and checksig2 to output the signal strength and signal quality numbers respectively. We can then probe the respective OID for these exec commands using mrtg or for testing purposes, you could use snmpget or snmpwalk.