score:1

Accepted answer

As you can see in the Highcharts Gantt source code: https://github.com/highcharts/highcharts/blob/47dfb49f7b6053178c92d24b34b5f3e7ddd8770b/ts/Series/Gantt/GanttSeries.ts#L202

shape for milestone can not be set through API options.

The simplest solution is to replace diamond function with the built in triangle function.

(function(H) {
    H.Renderer.prototype.symbols.diamond = H.Renderer.prototype.symbols.triangle
})(Highcharts);

Live demo: https://jsfiddle.net/BlackLabel/9zuodLjt/

Docs: https://www.highcharts.com/docs/extending-highcharts/extending-highcharts


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