If they hold that stance, the graph should not use 'Europe' when they essentially mean 'EU member nations', especially if they omit the UK for the reasons you state as the UK is a part of Europe.
Edit: not sure how difficult this is to understand. Europe is a continent, which includes the UK. The chart says Europe, not EU, whilst omitting a country within Europe.
As a home user the OS thing is preference, some prefer Windows, some Mac, some Linux, etc.
Your post however raises a good point, and it certainly makes me form an opinion in a greater context. Thanks for making me think about this, genuinely - it's good to have opinions challenged.