code: public void scale(double delta, ZoomOption option, Coordinate center) {
  		setCurrentScale(viewState.getScale() * delta, option, center);
  	}
docs: Adjust the current scale on the map by a new factor, keeping a coordinate in place.
  
  @param delta Adjust the scale by factor "delta".
  @param option zoom option
  @param center Keep this coordinate on the same position as before.