The evidence is the amount of money that has been spent on that research - it's a tiny drop.
Large scale research projects are required to probe science at this level - look at the development of fission reactors, for example. The money poured into that was vast, and it cracked the fundamental engineering problems associated with it.
Fusion power is not a theoretical concept - it happens all the time (and life on earth is reliant on it), but the practical challenges are large. The lines on that graph are obviously projections, but they are projections based on the science and engineering of the time as it pertained to fusion science. They weren't just "made up", and they do not take unforeseen circumstances into account, but they are based on the costs of solving the challenges inherent in fusion power production which were known at the time the graph was made.
You don't believe that fusion researchers are doing anything useful, so it's clear you don't understand how science works, so this is likely lost on you, but the amount of money on that graph in total since 1978 is so small that it is laughable, and yet here we are. It outlines one of the main problems with large scale science - that short sighted people such as yourself consider pure research to be "harmful" because it isn't immediately profitable or an obvious path to near-term profits.
From my perspective, the 20 billion per year air conditioning the desert in Afghanistan is wasted - what exactly has the war in Afghanistan accomplished? Apart from destabilising the region, increasing xenophobia, damaging the USA's reputation and giving a few people some closure because some terrorists who weren't from either of the two countries you invaded in response flew some planes into a couple of buildings in NYC.
Solving fusion power will change the face of civilisation and is an almost-necessary step in transitioning into an era where the bulk of our energy doesn't come from fossil sources (it could be done with purely fission power too, but again, PR issues and funding problems dog it). The worst part is we could have already solved it by now had we actually spent any reasonable amount of money on it. If it had been funded at 5 billion dollars per year since 1976 then you could have had twenty thousand simultaneous fusion power research programs running over those 40 years for every eight-year Iraq war (using low estimate for the cost of the war).
The point being, fusion power is being funded for peanuts, and even the "aggressive funding" is a tiny amount.