Today I realized that many of my students in an upper-division undergraduate (projective) geometry class don't really understand how to use pictures in proofs. Some draw no pictures at all, making their proofs nearly impossible to read; some oppositely draw a picture but don't explain it at all; and others try to explain their picture but get confused about things like which elements of the picture are givens and which are constructed.
Now that I've noticed this problem, it seems obvious and that I should have expected it. Probably no one has ever taught them how to use pictures in proofs! I can't remember ever seeing this topic discussed in a logic textbook or an introduction-to-proofs class. And the correct use of pictures in proofs is actually a very subtle thing, e.g. how a picture should be "generic" so as to not give false intuitions.
Can anyone point to any good resources (books, papers, web pages, videos, whatever) that teach students how to use pictures in proofs?