title: Tooltip for D3 Heatmap canvas
body: <p>Here i'm working on <a href="http://bl.ocks.org/robnagler/e245b69c473da73dfb85" rel="nofollow">D3 Heatmap with resettable zoom</a>, but i want to add tooltip to view the intensity count. With help of <a href="http://labratrevenge.com/d3-tip/javascripts/d3.tip.v0.6.3.js" rel="nofollow">D3 tip</a>, i tried to add tooltip, but don't know how to get the intensity count from canvas, where the heatmap is drawn as an image data. Please check out my <a href="https://jsfiddle.net/NayanaDas/6qmeb7oe/2/" rel="nofollow">fiddle</a>.</p>
  
  <p>Code used for adding tooltip:</p>
  
  <pre><code>var tip = d3.tip()
  .attr('class', 'd3-tip')
  .offset([-10, 0])
  .html(function (d) {
      return "tooltip";
  })
  svg.call(tip);
  
  svg.on('mousemove', tip.show);
  svg.on('mouseout', tip.hide);
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>
  
  <p>Thank you in Advance.</p>