使用svg.js扩展插件svg.draggable.js在控制台报错？TypeError: rect.draggable is not a function

var draw = SVG().addTo('#drawing').size('100%', 300);
        var rect = draw.rect(100, 100);
        rect.draggable();

http://www.htmleaf.com/html5/SVG/201501311304.html