Force Layout with Tooltips

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.link {
  stroke: #777;
  stroke-opacity: 0.2;
  stroke-width: 1.5px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var width = 960,
    height = 500

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

var force = d3.layout.force()
    .gravity(.05)
    .charge(-240)
    .linkDistance(60)
    .size([width, height]);

d3.json("readme.json", function(error, json) {
  if (error) throw error;

force
      .nodes(json.nodes)
      .links(json.links)
      .start();

var link = svg.selectAll(".link")
      .data(json.links)
    .enter().append("line")
      .attr("class", "link");

var node = svg.selectAll(".node")
      .data(json.nodes)
    .enter().append("g")
      .attr("class", "node")
      .call(force.drag);

node.append("image")
      .attr("class", "circle")
      .attr("xlink:href", "mbostock.jpg")
      .attr("x", "-10px")
      .attr("y", "-10px")
      .attr("width", "20px")
      .attr("height", "20px");

node.append("title")
      .text(function(d) { return d.name; });

force.on("tick", function() {
    link.attr("x1", function(d) { return d.source.x; })
        .attr("y1", function(d) { return d.source.y; })
        .attr("x2", function(d) { return d.target.x; })
        .attr("y2", function(d) { return d.target.y; });

node.attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });
  });
});

</script>

https://d3js.org/d3.v3.min.js