extending Raphael.js: el.draggable()

/* 
    HELPER - extend raphael: draggable elements
*/

Raphael.el.draggable = function(){
    //state
    this.attr({cursor: "move"});
    this.drag(
        //move
        function(dx,dy){
            this.attr({x: this.ox + dx, y: this.oy + dy}); 
        },
        //start
        function(){
            this.ox = this.attr("x");
            this.oy = this.attr("y");
            //state
            this.attr({opacity: 0.4});
        }, 
        //stop
        function(){
            //state
            this.attr({opacity: 1});
        });
};