In this case it is the graph makers laziness to not find extra stats on the UK. On the other part I believe the UK has the choice to join again, especially on side project as Eurostat. If a European country doesn’t want to join on European initiatives then I’m bit sure if we should treat them specially. The choice is theirs and it was clearly made.

The data is provided by Eurostat. An EU organisation, they are not going to fund numbers for a country not in the EU anymore (some other countries might still join for various reasons.)