Make an element follow a path

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8"/>
<style type="text/css">
  #line{
    width: 100%;
    margin: 20px 0;
    height: 300px;
    background: #eee;
  }
  path {
    stroke: steelblue;
    stroke-width: 1;
    fill: none;
  }
  button {
    margin: 20px 0 0 20px;
  }
</style>
</head>
<body>
  <button id="randomize">Create random line</button>
  <div id="line"></div>
  <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/d3@2.5.1/d3.js"></script>
  <script type="text/javascript">
    var svg = d3.select("#line").append("svg:svg").attr("width", "100%").attr("height", "100%");
    var data = d3.range(50).map(function(){return Math.random()*10})
    var x = d3.scale.linear().domain([0, 10]).range([0, 700]);
    var y = d3.scale.linear().domain([0, 10]).range([10, 290]);
    var line = d3.svg.line()
      .interpolate("cardinal")
      .x(function(d,i) {return x(i);})
      .y(function(d) {return y(d);})

var path = svg.append("svg:path").attr("d", line(data));
    var circle = 
        svg.append("circle")
          .attr("cx", 100)
          .attr("cy", 350)
          .attr("r", 3)
          .attr("fill", "red");

var pathEl = path.node();
    var pathLength = pathEl.getTotalLength();
    var BBox = pathEl.getBBox();
    var scale = pathLength/BBox.width;
    var offsetLeft = document.getElementById("line").offsetLeft;
    var randomizeButton = d3.select("button");

svg.on("mousemove", function() {
      var x = d3.event.pageX - offsetLeft; 
      var beginning = x, end = pathLength, target;
      while (true) {
        target = Math.floor((beginning + end) / 2);
        pos = pathEl.getPointAtLength(target);
        if ((target === end || target === beginning) && pos.x !== x) {
            break;
        }
        if (pos.x > x)      end = target;
        else if (pos.x < x) beginning = target;
        else                break; //position found
      }
      circle
        .attr("opacity", 1)
        .attr("cx", x)
        .attr("cy", pos.y);
    });

randomizeButton.on("click", function(){
      data = d3.range(50).map(function(){return Math.random()*10});
      circle.attr("opacity", 0)                                     
      path
        .transition()
        .duration(300)
        .attr("d", line(data));
    });
  </script>
</body>
</html>

Modified http://mbostock.github.com/d3/d3.js?2.5.0 to a secure url

https://mbostock.github.com/d3/d3.js?2.5.0