title: NVD3 TooltipContent Does not work
body: <p>I am using NVD3 library for my project and i have written following code.</p>
  
  <pre><code>var chart = nv.models.lineChart()
              .useInteractiveGuideline(true)
              .margin({top: 50, right: 50, bottom: 50, left: 50})
              .tooltipContent(function (key, y, e, graph) {
                  console.log("helo");
                  return "hello";
              });
  </code></pre>
  
  <p>Expected output should be to show hello on mouse over.
  But i dont get that, instead i get the default tooltip.</p>
  
  <p>Please let me know the mistake i am doing.</p>