You don't need a standard for ad-hoc whiteboard diagrams or expositional figures. You're comparing apples and oranges because what you are talking about needing

You don't need a standard for ad-hoc whiteboard diagrams or expositional figures. You're comparing apples and oranges because what you are talking about needing this level of precision in is more "blueprints", not ad-hoc diagrams. And, while it's my opinion only, blueprints for software are best written in prose. For C++ for example, I think the best language to use is concise, precise, well written, C++. And so on.