title: Show d3.layout.force link tooltip on mouse over
body: <p>How do I implement tooltips on mouse over for links in a <code>D3</code> directed graph layout? I'm adapting the <code>D3</code> force example, so setting up node tooltips was straightforward using code like this:</p>
  
  <pre><code>    node.append("title")
          .text(function(n) {
              return n.id;
          });
  </code></pre>
  
  <p>Trying a similar technique with the links didn't result in mouse over tool tips:</p>
  
  <pre><code>    var link = svg.selectAll("line.link")
          .data(json.links)
          .enter().append("line")
          .attr("class", "link")
          .style("stroke-width", function(d) {
              return 4;
          });
  
  link.append("title")
      .text(function(n) {
              return n.info;
          });
  </code></pre>