It is the complex plane, x is real, y is imaginary. the author should have clarified this, you are correct, as the author also makes clear he is writing for a

It is the complex plane, x is real, y is imaginary. the author should have clarified this, you are correct, as the author also makes clear he is writing for a non-technical audience. He probably forgot to mention it because the concept is so fundamental to signal processing that he assumed it was common knowledge.