sclass ImageSurfaceWithXMarkers extends ImageSurface {
  new Set<Float> markers;
  
  *(BufferedImage img, Collection<Float> _markers) {
    super(img);
    addAll(this.markers, _markers);
    overlay = voidfunc(Graphics2D g) {
      g.setColor(Color.red);
      for (float x : markers) {
        Pt p = pointToComponentCoordinates(x, 0);
        drawLine(g, p.x, 0, p.x, getHeight()-1);
      }
    };
  }
}