As for a QLN plot, consider it to be a snapshot of the entire frequency band in use by the xDSL service.
Most circuits in the UK will show a peak corresponding to the 198 kHz signal broadcast from the
Droitwich transmitter, whilst circuits far north of the border into Scotland will show an identical peak originating from the Burghead and the Westerglen transmitters.
In the past, some of us have made attempts to geo-locate fellow
Kitizens by cross-referencing the peaks originating from licensed broadcast transmissions. In my own case, it can be quite clearly seen that
The Cattery is located in East Anglia.
Radio frequency ingress is inversely proportional to the AC balance of an xDSL circuit, amongst many other things.
An ideal QLN plot would be fairly flat, with a noise floor around -140 dBm/Hz. However that ideal is very rarely seen . . .
As examples of QLN plots that are "real world" but away from the ideal -- with the xDSL circuits operating quite normally -- take a look at the graphs from
N*Star and
tommy45, attached below.