ChartJS: Draw line between two data points on hover

Question

Hi everyone, I have created the chart shown in the image.

Does anyone know how to draw a line between dataset of same index when trying to hover on either data (the red line).

I'm using ChartJS on VueJS component

Answer 1

The Plugin Core API offers a range of hooks that may be used for performing custom code. You can draw the line directly on the canvas using CanvasRenderingContext2D.stroke().

In below code snippet, I use the afterDraw hook and draw the line in case the tooltip would be shown otherwise. Please consult addPlugin from vue-chart.js documentation to learn how to add such inline plugin in Vue.js.

var chart = new Chart('canvas', {
  type: 'line',
  plugins: [{
    afterDraw: chart => {
      if (chart.tooltip._active && chart.tooltip._active.length) {
        const ctx = chart.ctx;
        ctx.save();
        const activePoint = chart.tooltip._active[0];
        const x = activePoint.tooltipPosition().x; 
        const yAxis = chart.scales['y-axis-0'];
        const value1 = chart.data.datasets[0].data[activePoint._index];
        const value2 = chart.data.datasets[1].data[activePoint._index];
        const y1 = yAxis.getPixelForValue(value1);
        const y2 = yAxis.getPixelForValue(value2);
        ctx.beginPath();
        ctx.moveTo(x, y1);
        ctx.lineTo(x, y2);
        ctx.lineWidth = 2;
        ctx.strokeStyle = 'green';
        ctx.stroke();
        ctx.restore();
      }
    }
  }],
  data: {
    labels: ['A', 'B', 'C', 'D', 'E', 'F'],
    datasets: [{
        data: [13, 10, 12, 13, 9, 12],
        backgroundColor: 'rgba(255, 99, 132, 0.2)',
        borderColor: 'rgb(255, 99, 132)',
        fill: false
      },
      {
        data: [10, 7, 9, 10, 6, 9],
        backgroundColor: 'rgba(255, 159, 64, 0.2)',
        borderColor: 'rgb(255, 159, 64)'
      }
    ]
  },
  options: {
    legend: {
      display: false
    },
    tooltips: {
      enabled: false
    },
    scales: {
      yAxes: [{
        ticks: {
          beginAtZero: true
        }
      }]
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.4/Chart.min.js"></script>
<canvas id="canvas" height="90"></canvas>