Hold down CMD+Option, and then press down with two fingers UNTIL THE TRACKPAD CLICKS, and zoom will do a great job of zooming the way you move your fingers: if you pinch up and down, it will zoom vertically. If you pinch side-to-side, it will zoom horizontally. It becomes super-intuitive, and I don't know why it's not the default.
Posting this because every time I have to intensive work in melodyne, I go through hell trying to remember/google how to get zoom to work on just one axis. Maybe I am the only one with this problem, but I swear the first 30 pages of Google results are "how to zoom both horizontally and vertically at the same time", when what I want is how to do one or the other,