Eyeballing that graph, the two trend lines intersect at week 17 in 2014, i.e., end of April (I think each fine grey line stands for two weeks). Even at its best, mid-2015, Remain was at 45% and Leave at 38%. Given the wild variation in polling outcomes, that's firmly within the margin of error. You shouldn't oversell this point. Remain never seemed a done deal, and specifically when you look at the polls in Q2 of 2014 - i.e., at the time of the Indyref campaign - you see several poll outcomes where Leave won. That certainly doesn't say "no is never gonna happen".