After playing on and off in Interface Builder trying everything again and again, I've started to search more on StackOverflow and I've stumbled upon this question: Application crashes when working with core plot
Pretty weird, but it wasn't and IB issue (I was sure I was doing the right thing in IB, I've done it a thousand times before), but a linker issue. I don't know why, but I had to add -ObjC
to linker flags, and it worked.