Bernt,
Are you a lefty! We don't allow lefties here! Just kidding- so following the context of the thread- anywhere in your post where you said right arm - I'm putting in left arm.
Thanks,
Mike O.
Further, that was my initial thought - that the left arm could be on plane at one point (parallel to the ground)- but in a normal golf swing it never is on plane (with the exception of a zero #3 accumulator).
Because the left arm starts at address steeper than the plane and then at the top is flatter than the plane does not mean that somewhere in between it is on plane. You'll see or verify that with any swing sequence where the left arm is parallel to the ground and the left shoulder is no where near on-plane.