docs: @Override
    public Element useMarker(SVGPlot plot, Element parent, double x, double y, int stylenr, double size) {
      Element marker = plot.svgCircle(x, y, size * .5);
      final String col;
      if(stylenr == -1) {
        col = dotcolor;
      }
      else if(stylenr == -2) {
        col = greycolor;
      }
      else {
        col = colors.getColor(stylenr);
      }
      SVGUtil.setStyle(marker, SVGConstants.CSS_FILL_PROPERTY + ":" + col);
      parent.appendChild(marker);
      return marker;
    }