As a short follow up to my earlier posting - If you did decide to decline the vertical wall 90 degrees until it was directly east/west, the analemmatic dial on its surface would become what is known as a Parent dial - after Antoine Parent who described it in 1701. The ellipse will have collapsed into a straight line segment in this case. As you can see, the fact that the wall declines is very important and it does change the shape of the ellipse and the latitude at which the parallel horizontal plane will be found.
Fred Sawyer
