Thanks a lot for taking the time to reply. Unfortunately, I think I didn't explain very well what I meant. It's probably best demonstrated here:
https://youtu.be/6dhZ8P0Pj24?t=67
(different software)
At about 1:07 he demonstrates this behaviour:
Drag up/down for vertical zoom, drag left/right for horizontal zoom.
So they can happen at the same time but not necessarily in equal measures, if that makes sense.
Together with hand scroll (which I know how to get) I find this a wonderfully intuitive way to navigate big time lines.