code: public Marker attachLabel(MapLabel mapLabel) {
          optMapLabel = Optional.of(requireNonNull(mapLabel));
          mapLabel.setMarker(this);
          mapLabel.visibleProperty().bind(visibleProperty());
          mapLabel.positionProperty().bind(positionProperty());
          return this;
      }
docs: attaches the MapLabel to this Marker
  
  @param mapLabel
  the MapLabel to attach
  @return this object
  @throws NullPointerException
  of mapLabel is null