Reading https://www.amcharts.com/docs/v4/concepts/formatters/formatting-numbers/, it seems to me that to display data as percentages you only need to write :
chart.numberFormatter.numberFormat = "%";
However, not only it doesn't work, but it also hides all data-related information. It seems to be a bug because other formats such as "#.##" work fine.
Here is a stackblitz : https://stackblitz.com/edit/typescript-1arbck
Any help is appreciated !
If you want to show the percentage value in chart, then calculate the percentage value at front/back end and the show the value in chart. Sometimes Amcharts percentage calculation will not match with your calculated percentage value. Also, use chart.numberFormatter.numberFormat = "#.## '%'"; to show the percentage sign.