Sort of, kind of, but it's a stretch. "Up and down" on a page are not oriented according to gravity, but according to the top and bottom of the page. If you say "Well, we'll make the z-axis vertical because vertical is perpendicular to the plane of the desk, and that's depth", and then you put the z-axis toward the top of the page, you're using two completely different definitions of "vertical" in the same sentence.