[Chartjs]-ChartJS add custom tooltip to a stacked bar chart

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The trick is that the callback function must collect and return an array of labels only once but not for every dataset. 

callbacks: {
  label: (tooltipItem, data) => {
    if (tooltipItem.datasetIndex > 0) {
      return null;
    }

There were also issues with accessing wrong objects/properties when composing the main label.

Please have a look at your amended code below.

new Chart("canvas", {
  type: 'bar',
  data: {
    labels: ["January", "February", "March", "April", "May", "June", "July"],
    datasets: [{
      label: 'Task 1',
      backgroundColor: "rgba(220,220,220,0.5)",
      data: [50, 40, 23, 45, 67, 78, 23]
    }, {
      label: 'Task 2',
      backgroundColor: "rgba(151,187,205,0.5)",
      data: [50, 40, 78, 23, 23, 45, 67]
    }, {
      label: 'Task 3',
      backgroundColor: "rgba(82,154,190,0.5)",
      data: [50, 67, 78, 23, 40, 23, 0]
    }]
  },
  options: {
    title: {
      display: true,
      text: "Chart.js Bar Chart - Stacked"
    },
    tooltips: {
      mode: 'label',
      callbacks: {
        label: (tooltipItem, data) => {
          if (tooltipItem.datasetIndex > 0) {
            return null;
          }
          var tasks = data.datasets[tooltipItem.datasetIndex].label;
          var valor = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];
          var count = data.datasets.length;
          var total = 0;
          var label = '';
          for (var i = 0; i < data.datasets.length; i++) {
            total += data.datasets[i].data[tooltipItem.index];
          }
          label += "Total : " + total + " (";
          for (var i = 0; i < data.datasets.length; i++) {
            if (label.endsWith(",")) {
              label += " ";
            }
            label += data.datasets[i].label + " - $" + data.datasets[i].data[tooltipItem.index].toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",") + ",";
          }
          label = label.slice(0, -1) + ")";
          return [label, "Count : " + count];
        }
      }
    },
    responsive: true,
    scales: {
      xAxes: [{
        stacked: true,
      }],
      yAxes: [{
        stacked: true
      }]
    }
  }
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.bundle.min.js"></script>
<canvas id="canvas" height="100"></canvas>

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