function radialScale(xfactor, yfactor = xfactor) {
  return (onRefs, onOffsets, offOffsets) => {
    const scaledOnRefs = onRefs.map(pt => 
      [ (pt[0] - 500) * xfactor + 500, (pt[1] - 500) * yfactor + 500 ]);
    return [ scaledOnRefs, onOffsets, offOffsets ];
  }
}
Radial scaling for tracking adjustment
@param { number } xfactor Radial scaling factor for x axis 
@param { number } yfactor Radial scaling factor for y axis 
@returns { Filter }