Force Layout with Mouseover Labels

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

.link {
  stroke: #777;
  stroke-opacity: 0.3;
  stroke-width: 1.5px;
}

.node circle {
  fill: #ccc;
  stroke: #000;
  stroke-width: 1.5px;
}

.node text {
  display: none;
  font: 10px sans-serif;
}

.node:hover circle {
  fill: #000;
}

.node:hover text {
  display: inline;
}

.cell {
  fill: none;
  pointer-events: all;
}

</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(0.1)
    .charge(-120)
    .linkDistance(30)
    .size([width, height]);

var voronoi = d3.geom.voronoi()
    .x(function(d) { return d.x; })
    .y(function(d) { return d.y; })
    .clipExtent([[0, 0], [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);

var circle = node.append("circle")
      .attr("r", 4.5);

var label = node.append("text")
      .attr("dy", ".35em")
      .text(function(d) { return d.name; });

var cell = node.append("path")
      .attr("class", "cell");

force.on("tick", function() {
    cell
        .data(voronoi(json.nodes))
        .attr("d", function(d) { return d.length ? "M" + d.join("L") : null; });

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; });

circle
        .attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });

label
        .attr("x", function(d) { return d.x + 8; })
        .attr("y", function(d) { return d.y; });
  });
});

</script>

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