Those QLN graphs don't help much, as they only cover the ADSL tones (tone 300 is around 1.3MHz) rather than the full set needed for VDSL2 (up to 17MHz, or around 4000 tones).
I can't tell you what to do with DSLstats - I monitor mine using the other package (HG612 Modem Stats) from the same forum, and have recently started using mydslwebstats too: http://forum.kitz.co.uk/index.php?board=46.0
QLN does indeed show you the "Quiet Line Noise" - which is the amount of "noise" measured on your line prior to the modem squawking at each other to synchronise.
A level of -140 would appear to be the base for a very quiet line at all frequencies. I have seen this *once* on my line, while the rest of the time I see a regular pattern with values between -120 and -140. For the one "odd" time, the best guess is that the DSLAM restarted that time, so the QLN measurement was made with no neighbouring VDSL2 modems running.
Other ways to look at what has happened on your line:
- HLog graph: shows the quality of your line. A non-"smooth curve" to the line might suggest problems with the wiring topology, such as a bridge tap, while significant changes from sync to sync might suggest an intermittent problem in some joints.
- SNR per tone: big changes (drops) here might suggest an increase in noise - which could be plain interference, or crosstalk.
- Bits per tone: should reflect the SNR per tone graph.
Over a year ago, I made a post on a simiar vein, where I animated my 18-month history of these graphs. You can see the post & animations here:
Result: There may have been issues with the modem, and what it was reporting at the time. The behaviour I saw disappeared when the modem firmware was updated in October 2013.
I now run at a speed of 79Mbps, and an SNRM of 6dB; the bottom line is that you *will* get gradually-reducing SNRM values over time. You will also get a gradually-increasing rate of errors, until DLM is triggered; I am now in the region where this can happen, and has happened (I get around 72Mbps during DLM intervention).
Personally, I think that a drop of ~1.5dB is not actually that much, and you can expect much more. In Ireland, where they have turned on vectoring (to counter crosstalk), people have seen increases in SNRM of 10dB or more.