javascript - Chart.js - show tooltip when hovering on legend

Question

I recently upgraded my version of Chart.js from v2.3 to v2.7.1, which broke an existing functionality where the specified segment in a doughnut chart would be active (hover color, tooltip shown) when the user hovered over the corresponding legend item. That code looked like this:

 var " + ClientID + @" = new Chart(" + ClientID + @"CTX, {
    data: { ... },
    options: {
        legend: {
            onHover: function(evt, legendItem) {
                var index = " + ClientID + @".data.labels.indexOf(legendItem.text);
                if (" + ClientID + @".data.datasets[0].data[index] > 0) {
                    var metaData = " + ClientID + @".getDatasetMeta(0);
                    var activeSegment = metaData.data[index];
                    " + ClientID + @".tooltipActive = [activeSegment];
                    " + ClientID + @".active = [activeSegment];
                }                                   
            },
        }
    }
});

Looking through the Chart.js file and documentation, it looks like the tooltipActive property has been completely removed, thus breaking the legend hover functionality. I looked through the release notes and PRs on the Chart.js git but couldn't find where this was noted as a breaking change, or any mention of it whatsoever. I have to upgrade versions of Chart.js for a separate change I'm making, so reverting back to v2.3 is not an option. Has anyone else run into this?

Answer 1

Here's a solution based on your approach that works with the current version of Chart.js (2.9.3).

Beside onHover, you also have to defined an onLeave callback function. This makes sure to hide the tooltip and revert the hover effect as soon as the mouse pointer leaves the legend label.

const chart = new Chart('chart', {
  type: 'doughnut',
  data: {
    labels: ['One', 'Two', 'Three'],
    datasets: [{
      data: [4, 5, 3],
      backgroundColor: ['rgba(255, 99, 132, 0.2)', 'rgba(255, 159, 64, 0.2)', 'rgba(54, 162, 235, 0.2)'],
      borderColor: ['rgb(255, 99, 132)', 'rgb(255, 159, 64)', 'rgb(54, 162, 235)'],
      hoverBackgroundColor: ['rgba(255, 99, 132, 0.4)', 'rgba(255, 159, 64, 0.4)', 'rgba(54, 162, 235, 0.4)'],
      borderWidth: 1,
      hoverBorderWidth: 3
    }]
  },
  options: {
    legend: {
      onHover: (evt, legendItem) => {
        const index = chart.data.labels.indexOf(legendItem.text);
        const activeSegment = chart.getDatasetMeta(0).data[index];
        activeSegment._model.backgroundColor = activeSegment._options.hoverBackgroundColor;
        activeSegment._model.borderWidth = activeSegment._options.hoverBorderWidth;
        chart.tooltip._active = [activeSegment];
        chart.tooltip.update();
        chart.draw();
      },
      onLeave: (evt, legendItem) => {
        const index = chart.data.labels.indexOf(legendItem.text);
        const activeSegment = chart.getDatasetMeta(0).data[index];
        activeSegment._model.backgroundColor = activeSegment._options.backgroundColor;
        activeSegment._model.borderWidth = activeSegment._options.borderWidth; 
        chart.tooltip._active = [];
        chart.tooltip.update();
        chart.draw();
      }
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.min.js"></script>
<canvas id="chart" height="80"></canvas>