The Y axis here is not an absolute international political compass. It measures which political party each person favors, and judging by that country's local standards categorizes that party as either left or right.

A rising number in the US chart means a larger number of people prefer democrats over republicans. It doesn't mean that people's stances are necessarily moving further left. Similarly, it's no coincidence that the inflection point where UK numbers rise by a lot correspond to Brexit: the party seen as responsible for the unpopular change lost a lot of support, but that doesn't mean the population has so sharply moved drastically more progressive in such a short time.