Detecting hover events over parts of a chart using Chart.js

Question

I've made a pie chart using Chart.js, and I'd like to detect when a segment is hovered over. I've found plenty of documentation regarding manipulating the tooltips that appear when hovering over segments, but nothing regarding doing something else when a tooltip would appear. Is this possible?

Answer 1

UPDATE 2020 : For Chartjs 3.5

Here is a quick fix that I used with Chartjs 3.5 to trigger events on hover over Doughnut or Pie parts. It relies on using the existing onHover option :

onHover : (event, activeElements) => {

    if (activeElements.length > 0) {

            // get active chart element
            let elt = activeElements[0];

            // retrieve element label
            let lbl = elt._model.label;

            // Get element value
            let elt_index = elt._chart.tooltip._data.labels.indexOf(lbl);
            let val = elt._chart.tooltip._data.datasets[0].data[elt_index];

           // trigger your event here :
           // ex for vuejs : this.$emit('element-hovered', { label : lbl, value : val });

       }
       else {
           // No active elements
       }

   }

Working fine so far :)

Answer 2

I know this has already been given an accepted answer, and im not sure if this does satisfy your use case but Chart js released an update (maybe a month ago or so) that allowed for custom tooltips. This allows for a custom function to run when a tooltip would normally have been drawn. They have an example in the samples section of git hub

in short you define a custom function

 Chart.defaults.global.customTooltips = function(tooltip){//do what you want}

here is the example they give in the samples with an extra bit of text added to an html tooltip. The only annoying thing I see is that don't provide the segment/point/bar that triggered this tooltip which would be really handy as then you could do some thing to the graph knowing this information but you are given the tooltip data which means you can do something with that instead.

Chart.defaults.global.customTooltips = function (tooltip) {
      // Tooltip Element
      var tooltipEl = $('#chartjs-tooltip');
      // Hide if no tooltip
      if (!tooltip) {
          tooltipEl.css({
              opacity: 0
          });
          return;
      }
      // Set caret Position
      tooltipEl.removeClass('above below');
      tooltipEl.addClass(tooltip.yAlign);
      // Set Text
      tooltipEl.html(tooltip.text+ " anythign custom you want");
      // Find Y Location on page
      var top;
      if (tooltip.yAlign == 'above') {
          top = tooltip.y - tooltip.caretHeight - tooltip.caretPadding;
      } else {
          top = tooltip.y + tooltip.caretHeight + tooltip.caretPadding;
      }
      // Display, position, and set styles for font
      tooltipEl.css({
          opacity: 1,
          left: tooltip.chart.canvas.offsetLeft + tooltip.x + 'px',
          top: tooltip.chart.canvas.offsetTop + top + 'px',
          fontFamily: tooltip.fontFamily,
          fontSize: tooltip.fontSize,
          fontStyle: tooltip.fontStyle,
      });
  };
  var pieData = [{
      value: 300,
      color: "#F7464A",
      highlight: "#FF5A5E",
      label: "Red"
  }, {
      value: 50,
      color: "#46BFBD",
      highlight: "#5AD3D1",
      label: "Green"
  }, {
      value: 100,
      color: "#FDB45C",
      highlight: "#FFC870",
      label: "Yellow"
  }, {
      value: 40,
      color: "#949FB1",
      highlight: "#A8B3C5",
      label: "Grey"
  }, {
      value: 120,
      color: "#4D5360",
      highlight: "#616774",
      label: "Dark Grey"
  }];

  var ctx1 = document.getElementById("chart-area1").getContext("2d");
  window.myPie = new Chart(ctx1).Pie(pieData);
  var ctx2 = document.getElementById("chart-area2").getContext("2d");
  window.myPie = new Chart(ctx2).Pie(pieData);

#canvas-holder {
       width: 100%;
       margin-top: 50px;
       text-align: center;
   }
   #chartjs-tooltip {
       opacity: 1;
       position: absolute;
       background: rgba(0, 0, 0, .7);
       color: white;
       padding: 3px;
       border-radius: 3px;
       -webkit-transition: all .1s ease;
       transition: all .1s ease;
       pointer-events: none;
       -webkit-transform: translate(-50%, 0);
       transform: translate(-50%, 0);
   }
   #chartjs-tooltip.below {
       -webkit-transform: translate(-50%, 0);
       transform: translate(-50%, 0);
   }
   #chartjs-tooltip.below:before {
       border: solid;
       border-color: #111 transparent;
       border-color: rgba(0, 0, 0, .8) transparent;
       border-width: 0 8px 8px 8px;
       bottom: 1em;
       content:"";
       display: block;
       left: 50%;
       position: absolute;
       z-index: 99;
       -webkit-transform: translate(-50%, -100%);
       transform: translate(-50%, -100%);
   }
   #chartjs-tooltip.above {
       -webkit-transform: translate(-50%, -100%);
       transform: translate(-50%, -100%);
   }
   #chartjs-tooltip.above:before {
       border: solid;
       border-color: #111 transparent;
       border-color: rgba(0, 0, 0, .8) transparent;
       border-width: 8px 8px 0 8px;
       bottom: 1em;
       content:"";
       display: block;
       left: 50%;
       top: 100%;
       position: absolute;
       z-index: 99;
       -webkit-transform: translate(-50%, 0);
       transform: translate(-50%, 0);
   }

<script src="https://raw.githack.com/chartjs/Chart.js/v1.1.1/Chart.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="canvas-holder">
    <canvas id="chart-area1" width="50" height="50" />
</div>
<div id="canvas-holder">
    <canvas id="chart-area2" width="300" height="300" />
</div>
<div id="chartjs-tooltip"></div>

Answer 3

The way I'm doing it is slightly more simple: assuming you already have some code defining the canvas like canvas = document.getElementById("chart"); and your pie chart is window.myPie. You can use the onmousemove javascript event, or jQuery hover

canvas.onmousemove = function(evt) {
    var el = window.myPie.getElementsAtEvent(evt);
    //do something with the el object to display other information
    //elsewhere on the page
}

In my case highlight a table row based on the value of el[0]._index