Hi
Gaps appear when there is too much noise at a particular frequency to fill the bin. If a particular bins SNR (note this is SNR not SNRM) is below 6dB then the router wont use that channel. This could be due to interference.. or it could be just too attenuated to receive a signal.
Whilst I understand the very basics of DMT modulation, Im not an expert as it also involves some further complicated
modulation techniques that would probably require a degree in the subject to understand fully
- but looking at your graph it looks like the decline is normal due to attenuation (loop loss/line length).
However why you get the one strong signal further on - sorry I dont know