Not sure I fully understand the question. The leading edge of the elevator has to be a tapering radius to match the taper of the stab. The dotted line represents the hinge line, IE, the center of the tapering radius. It is also probably a good indication of where the top skin of the elevator should stop in order to form the gap seal for a gap-less hinge line. Take a look at any good 3-view of a Zero and you will see that the visible separation between the stab and elevators is a straight line from tip to tip. The hinge line (your dotted line) also has to be a straight line from tip to tip. Thus, due to the tapering shape, the leading edge of the elevators is swept back as shown on the drawing. Hopefully this all makes sense and I apologize if I misunderstood the question.