If either sphere is dragged, the other sphere is dragged along to maintain the length of the gauge. If the "slider" cylinder is dragged, it cannot move off of the long cylinder or past its ends. The distance ratio of the "slider" along the long cylinder is used to compute density. If the long cylinder is dragged, the whole widget moves, maintaining their relative placements. Finally, if either of the resize cylinders is dragged, the length and the small cylinder moves to stay on the long cylinder and to maintain its previous distance ratio along the long cylinder.
