anchor.diff:  
     function onFrame(event) {
       mover_group.update_positions();
  +  }
  +
  +  function onMouseDown(event)  {
  +    mover_group.handle_mouse_down(event);
     }
   </script>
   <canvas id="visualization_canvas" resize>
  
positive.code:     update_rotation: function()  {
        this.path.rotate(this.rotate_speed);
      },
      move_to_destination: function()  {
        vector = this.destination - this.path.position;
        this.path.position += vector / 30;
  
negative.code:       //threshold is roughly 8 px
        if(segment_distance_sq < 65)  {
          //if below the threshold, make it much bigger
          this.increment_scale();
        } else {
          //if above a threshold, make it slightly smaller
          this.decrement_scale();
        }
  
        return vector;
  