Americans usually say "graphic".
When they say "graphical" they mean "the diagram" itself.
But when they want to refer to something relating to graphic arts, or something shown in picture, they say "graphic". I've heard British speakers sometimes use Graphical for Graphic. I'm not sure.
"Usage" is the way language is used and it is the example of language use. Proficient language learner is someone proficient in the usage.