until this feature is implemented, there is a small workaround: in set up mode, create a small png circle in photoshop, import it to the spine project and put it under a slot, then make the slot a child of the controller you want to check the arc for, and position it to the center of the controller. Now just activate ghosting, make sure you activate ghosting for selected only (the icon in the bottom, left to the lock). set number of frames for the before and after, and frame step to 1, make sure you use images, and solid colors. now in animate mode, select the controller, and move the timeline, you should she a good representation of the arc.. change ease in/out and keyframes to alter the arc/timing.