Typically the first one is more common, as the phrase "graduating in" usually would hint at a graduation year. It's not technically wrong, it's understandable and makes sense either way but the first one would sound more natural.

the former - I'm studying linguistics at U of C - graduating is quite another thing; I never heard of someone in the process of graduating, it is not an ongoing process, it is a true or false state - not yet graduated, just graduated.

Thank you very much!