public function svg($str_filename = null)
    {
        $str_svg = $this->svgForHtml5();
        $str_svg = '<?xml version="1.0" encoding="utf-8"?>'. "\n". $str_svg;

        if (!is_null($str_filename)) {
            file_put_contents($str_filename, $str_svg);
        }

        return $str_svg;
    }