function gravityAdjustment(factor) {
  return (onRefs, onOffsets, offOffsets) => {
    const f = (x) => (1-(1-x)**factor)**(1/factor);
    const scaledOnRefs = onRefs.map(pt => 
      [ pt[0], f(pt[1]/1000)*1000 ]);
    return [ scaledOnRefs, onOffsets, offOffsets ];
  }
}
Gravity control
@param { number } factor Gravity control factor
@returns { Filter }