I think the graph shows relative position of the planets from the sun, so the straight line is just a baseline.
Like if you jump up and down on a plane, and the graph just shows you moving a couple of feet up and down rather than including the entire altitude change of the plane itself





You could represent time as the distance from center to circumference, although that wouldn’t be as readily comprehensible at a glance. It’s more like the image just shows a chunk out of that sphere