__I cannot find how to animate lines as far as position of a *single anchor point* goes (not length)

I cannot find how to animate lines as far as position of a single anchor point goes (not length or overall position). There are tons of tutorials on how to animate along a path in hundreds of ways, but I'm having trouble finding out how to do something like I drew in this image (one anchor point stays in the same position throughout, while other moves/stretches)