Chart.js Multiple dataset

Question

I have a problem with Chart.js.

I want an an alert show me the ID value set in the dataset when I click on a point in the chart.

I have tried using getElementsAtEvent(evt);, but it doesn't work as I expected.

Can somebody help me? Thanks!

var ctx = document.getElementById("myChart");
var color = ["#ff6384", "#5959e6", "#2babab", "#8c4d15", "#8bc34a", "#607d8b", "#009688"];

var scatterChart = new Chart(ctx, {
  type: 'line',
  data: {
    datasets: [{
        label: 'Linea A',
        backgroundColor: "transparent",
        borderColor: color[1],
        pointBackgroundColor: color[1],
        pointBorderColor: color[1],
        pointHoverBackgroundColor: color[1],
        pointHoverBorderColor: color[1],
        data: [{
          x: "2014-09-02",
          y: 90,
          id: '1A'
        }, {
          x: "2014-11-02",
          y: 96,
          id: '2A'
        }, {
          x: "2014-12-03",
          y: 97,
          id: '3A'
        }]
      },
      {
        label: 'Linea B',
        backgroundColor: "transparent",
        borderColor: color[2],
        pointBackgroundColor: color[2],
        pointBorderColor: color[2],
        pointHoverBackgroundColor: color[2],
        pointHoverBorderColor: color[2],
        data: [{
          x: "2014-09-01",
          y: 96,
          id: "1B"
        }, {
          x: "2014-10-04",
          y: 95.8,
          id: "2B"
        }, {
          x: "2014-11-06",
          y: 99,
          id: "3B"
        }]
      }
    ]
  },
  options: {
    title: {
      display: true,
      text: 'Polveri',
      fontSize: 18
    },
    legend: {
      display: true,
      position: "bottom"
    },
    scales: {
      xAxes: [{
        type: 'time',
        time: {
          displayFormats: {
            'millisecond': 'MM/YY',
            'second': 'MM/YY',
            'minute': 'MM/YY',
            'hour': 'MM/YY',
            'day': 'MM/YY',
            'week': 'MM/YY',
            'month': 'MM/YY',
            'quarter': 'MM/YY',
            'year': 'MM/YY',
          },
          tooltipFormat: "DD/MM/YY"
        }
      }]
    }
  }
});

document.getElementById("myChart").onclick = function(evt) {
  var activePoints = scatterChart.getElementsAtEvent(evt);
  var firstPoint = activePoints[1];
  console.log(firstPoint._datasetIndex);
  console.log(firstPoint._index);
  var label = scatterChart.data.labels[firstPoint._index];
  console.log(scatterChart.data.datasets[0].data[0].id);
  var value = scatterChart.data.datasets[firstPoint._datasetIndex].data[firstPoint._index];
  if (firstPoint !== undefined)
    alert(label + ": " + value);
};

<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.29.0/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.5.0/Chart.js">
</script>
<canvas id="myChart" width="400" height="100"></canvas>

Answer 1

You have to change your label variable assignment from,

var label = scatterChart.data.labels[firstPoint._index];

To,

var label = scatterChart.data.datasets[firstPoint._index].label;

And you may also need to change your alert statment as,

alert(label + ": " + value.x);

Here is the working DEMO: https://jsfiddle.net/dt6c9jev/

Hope this helps!.