d3.js实现图形拖拽

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>拖拽操作</title>
</head>
<body>
<script src = "d3.js"></script>
<script>
  var width =2000,
      height = 2000;
  var svg = d3.select("body").append("svg")
      .attr("width",width)
      .attr("height",height)
  var color = d3.scale.category20()
  //定义拖拽函数
  var drag = d3.behavior.drag()
      .on("drag",dragmove)
  function dragmove(d){
    d3.select(this)
        .attr("cx", d.cx = d3.event.x)
        .attr("cy", d.cy = d3.event.y)
  }
 
  //绘制圆形
  var circles =[{cx:150,cy:200,r:30},
    {cx:250,cy:200,r:30}]
  svg.selectAll("circle")
      .data(circles)
      .enter()
      .append("circle")
      .attr("cx",function(d){return d.cx})
      .attr("cy",function(d){return d.cy})
      .attr("r", function (d) {
        return d.r
      })
      .attr("fill",color)
      .call(drag)
</script>
</body>
</html>