If you let a golf club hang from the grip end and also have a weighted string hanging from the grip end - the string would show you the longitudinal center of gravity of the golf club and the point in which it passes through the clubface is the sweetspot.
And . . .
Notice that the weighted string (plumb bob) hangs straight down. The Clubshaft does not.
It is this invisible "line of pull" that defines the Plane of the Stroke.