title: Tooltip not showing up d3.js
body: <p>I have simple tooltip question but I couldn't find the solution.
  The codes below draw a doughnut chart. When user mouseovers a segment of pie, the tooltip should pop up in the middle of doughnut. But I don't know why it does not work here. Can anyone help to point out the problem? Here is <a href="https://jsbin.com/dihovilabe/edit?html,css,js,output" rel="nofollow">JSbins</a> </p>
  
  <p>If I change the line 36 to <code>d3.select(#pieChart)</code>, the tooltip works. However, for some reasons, I want the tooltip to append on svg.</p>
  
  <p>Thanks a lot!</p>