In my experience, slow motion video makes it is easy to understand the relationship between getting off plane and the host of adjustments, including standing up, that occur thereafter. You'll often see a pivot that looks nice, then goes awry with the club getting off plane. This is immediately followed by a series of bizarre moves, mostly subconscious, attempting to get the club back in the right position through the impact zone, such as: standing on your toes, raising your shoulders, reverse weight shift, flipping your hands, violent spinning of the entire body and a variety of other not so great moves. You keep that sucker on plane and the same swing will lack the futile last second adjustments and suddenly look pretty good.
Personally, I think the straight plane line cannot be practiced too much. If you get that in decent shape and have any sort of idea what a good pivot should look like, you can start whacking the ball with a flat left wrist and lagging clubhead without too much difficulty. The fine tuning and clubface control are tricky, but a basic swing is not too hard once you can trace a straight plane line on a regular basis, IMHO.