)
    .enter().append("circle")
      .attr("class", "node")
      .attr("r", function(d) { return d.frequency * 500; })
      .style("fill", "#bbb")
      .call(force.drag)
      .on("dblclick", show_details)
      .on("mouseover", focus_topic)
      .on("mouseout", unfocus_topic);

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

  function show_details() {
    window.location = "topic/"+d3.select(this).select("title").text()+".html";
  }

  function focus_topic() {
    d3.select(this)