EDIT: Ah I see. DOCSIS shouldn't be on that graph. The graph relates to the tiers people are subscribed to, not limitations of the technology.
I see the graph as simply describing the speeds that subscribers get - including ADSL, ADSL2+, VDSL2 and DOCSIS - and *all* the limitations involved. Distance, money, or speed of rollout.
For DOCSIS, without distance limitations, it tells you the tiers that people have chosen - showing both the current tiers at the time (2014) and signs that some VM customers were left on old speed tiers. IIRC, the speed upgrades were an automatic thing, so it probably show "limitations" in VM's ability to roll out the upgrade quickly.
For ADSL and ADSL2+, where tiers don't exist, we see purely the distance limitation.
For VDSL2, we see a combination of the tiers and the distance limitation. I gimp'ed a version of the graph with a simplistic extrapolation of what
might be available if there were no tier choices, but I suspect it is a little wrong - that the percentage getting (just under) 80Mbps should run a touch higher.
It'd be great to see the impact of BT's announcement on this graph in this year's report, but Ofcom have this annoying habit of only publishing the really interesting graphs once, with no future follow-up.