I have looked if I could find a program that could convert the gpx file to a graph, but I haven't been able to find one that fits my needs. I have found a few that converted gpx to csv, and with some adjustments i have been able to make it save only the SSID and RSSI, but with large files they either crash or get very VERY slow.
I would like to see the signal strength of two networks over a day or a few days. Is it correct that inSSIDer can't open log files? I think that would be a great feature to add.