The charts seems a bit strange. What does England/Northern Ireland mean? Is it a combination of the two countries scores? Did they both score equally? Why leave out Scotland and Wales and not plot the UK?
This is a good question. The source is the "Survey of Adult Skills (PIAAC) (2012)." On the report they have a list of "National entities" (UK is not there) and "sub-national entities", where they have Flanders (Belgium), England (UK), Northen Ireland (UK), and England/N. Ireland (UK). I guess they don't have data for Scotland and Wales.
It doesn't even list all the OECD countries. This gives the false impression that US is at the bottom. Yep, it's worse than everyone better than it!
It does clearly include the average value and the USA value - even if it cherrypicks a few examples from the huge list of countries, these two numbers are enough to see that the impression isn't false.