public function addDot($x, $y, $w, $h, $rgb = [0, 0, 255])
    {
        // choose a color for the ellipse
        $ellipseColor = imagecolorallocate($this->image(), $rgb[0], $rgb[1], $rgb[2]);
        // draw the blue ellipse
        imagefilledellipse($this->image(), $x, $y, $w, $h, $ellipseColor);
    }