Chart.js does not call plugins.datalabels.formatte

Question

I wish to draw the percentages on the pie pieces of a Chart.js pie chart, in my Ionic 4 / Angular application.

My Chart.js version is 2.8.0

I have the following code I found here, where I can see it works. But when I add it, my formatter() is just not called (I have added a breakpoint and it is just not called)

        public ngOnInit() : void {
        this.data = {
          datasets: [{
            data: [10, 20, 30, 50],
            backgroundColor: [
              'green',
              'rgba(54, 162, 235, 0.2)',
              'rgba(255, 206, 86, 0.2)',
              'rgba(75, 192, 192, 0.2)',
              'rgba(153, 102, 255, 0.2)',
              'rgba(255, 159, 64, 0.2)'
            ],
          }],

          labels: [
            'Red',
            'Yellow',
            'Blue',
            'another'
          ]
        };

        let options = {
          responsive: true,
          maintainAspectRatio: false,
          legend: {
            position: 'bottom',
            boxWidth:10
          },
          tooltips: {
            enabled: false
          },
          plugins: {
            datalabels: {
              formatter: (value, ctx) => { // this is never called
                return 'hello';
                let sum = 0;
                let dataArr = ctx.chart.data.datasets[0].data;
                dataArr.map(data => {
                  sum += data;
                });
                let percentage = (value * 100 / sum).toFixed(2) + "%";
                return percentage;


              },
              color: 'black',
            }
          }
        };
        this.chart = new Chart(this.chartRef.nativeElement, { 
          type: 'pie',
          data: this.data,
          options: options
        });
      }

Would anyone have any idea why this is not working for me. Apart from that, most other things on the charts seem to work fine.

Thanks in advance for any help.