The line from our nocking point.......to the "spot" at 20yds would be another straight line.
No, the illustration that Bry presents is accurate, rudimentary but accurate. What it doesn't show is nock travel would create a line compensating for the arch the arrow must travel to drop into the target. If you followed the arrow path in flight with a line, nock travel would would rise because the bow/arrow is angled up, not nock to rest but because human influence.