title: Chart.js getElementsAtEvent is returning empty array for line chart
body: <p>We are using Chart.js 2.1.4 in a project we are working on.  When trying to get the point where a line chart was clicked on, the getElementsAtEvent function is returning an empty Array.  I understand this is expected behavior if the user clicks arbitrarily on the chart, but it is also occurring even if the user clicks on a datapoint.  Here is our onClick event handling:</p>
  
  <pre><code>options: {
      onClick: function(evt){
          var firstPoint = this.getElementAtEvent(evt);
  
          if (firstPoint) {
              console.log(firstPoint);
          } else {
              console.log('No point returned');
          }
      },
  </code></pre>
  
  <p>In the above code, firstPoint is an Array object with no items in it.</p>