.attr("class", "bar")  //为元素添加class属性,<div class="bar"></div>

.classed("bar", true)   //添加class "bar"

.classed("bar", false)  //删除class "bar"

var dataset = [ , , , ,  ];

d3.select("body").selectAll("div")

    .data(dataset)

    .enter()

    .append("div")

    .attr("class", "bar");

//css

div.bar {

    display: inline-block;

    width: 20px;

    height: 75px;   /* We'll override height later */

    background-color: teal;

}

.style("height", function(d) {

    return d + "px";

});

.style("height", function(d) {

    var barHeight = d * ;  //Scale up by factor of 5

    return barHeight + "px";

});  

//并设margin-right:2px;