几个非常有用的links: [1] three little circles. http://bost.ocks.org/mike/circles/ [2] How selection works. http://bost.ocks.org/mike/selection/ [3] Thinking with join. http://bost.ocks.org/mike/join/ 具体而言,data()是用来将DOM element与数据进行绑定起来,其第二个参数可以用来specify当前的每