title: Chart.js - Datalabels not showing using formatter
body: <p>I am trying to use the datalabels' plugin from Chartjs but the labels don't display on the graph at all! I am using >= 2.7.0 (actually 2.9.0) version of Chart.js. </p>
  
  <p>This is my code : </p>
  
  <pre><code>var ctx = this.pieChartRef.nativeElement;
      this.pieChart = new Chart(ctx, {
        type: 'pie',
        data: {
          labels: Object.keys(pieData),
          datasets: [
            {
              data: Object.values(pieData),
              backgroundColor: random,
            },
          ],
        },
        options: {
          legend: {
            position: 'bottom',
            display: true,
          },
          plugins: {
            datalabels: {
              color: '#fff',
              display: true,
              formatter: function (value, ctx) {
                let sum = 0;
                let dataArr: number[] = ctx.chart.data.datasets[0]
                  .data as number[];
                dataArr.map((data) =&gt; {
                  sum += data;
                });
                return ((value * 100) / sum).toFixed(2) + '%';
              },
            },
          },
        },
      });
  </code></pre>
  
  <p>No datalabel shows up, even if I replace the formatter code by a simple <code>return value + '%';</code>... Could you please help me, I am stuck ! </p>
  
  <p>Thank you in advance </p>