"Only If" Diagram

[DavidS674]

From the Logical Reasoning book:

Given: Juan plays drums only if Yohei plays guitar.

Given Diagram: ?
Contrapositive Diagram: ?

It says that the correct answers are:
GIven: JD -> YG
Contra: -YG-> -JD

It makes sense when I think about it logically, but I don't get the formal logic of it. My understanding was that if you had "x only if y" then the diagram is y->x. Very confused about how this works and I've reread the book explanations a bunch of times.

[smiller]

The statement "x only if y" translates to x → y. For example, "Sam eats spinach only if it is raw" means that if Sam is eating spinach then it must be raw (Sam eats spinach → raw).