What does "diagram" mean in translations from Chinese?
I've come across "diagram" in translations from Chinese weiqi sources, but I'm not sure what is meant by this. Would the usual term in English be something like game record/kifu, opening patterns/fuseki, patterns generally or an idea that's not got an easy translation?
Yes, 圖 is a very general word, not just for weiqi. It means anything that can be shown as "image-like", anything that you can draw. For game record/kifu we usually use the word 譜 or more specifically 棋譜 (in fact this is how the word kifu came from ki = 棋, fu = 譜). But you can also use a more general word to describe a kifu showing as an image (i.e. 圖). And in ancient time, this word would generally mean a game position.
You can also use more descriptive adj words to narrow it down, like 變化圖 (which would means variations image/diagram), what's important is the adj words before it, not the word 圖 itself that describes the meaning. (like 布局圖 opening diagram -> fuseki). But adj in general can be "omitted", hence we can write 圖 1 (diagram 1) 圖 2 (diagram 2) instead of using the whole word 變化圖1, 變化圖2 (if we already know the context is about variations).
If you want to translate it accurately, you need the context within (or even before and after) a paragraph in order to get its precise meaning.