如何导出透明背景的amChart？

var chart = am4core.create("chartdiv", am4charts.XYChart);
chart.paddingRight = 20;
chart.backgroundColor = am4core.color("rgba(0, 0, 0, 0)"); // 设置背景色为透明
// 其他配置项...

chart.exporting.menu = new am4core.ExportMenu();
chart.exporting.menu.items = [{
  "label": "...",
  // 其他导出选项...
}, {
  "label": "导出为PNG",
  "menu": [{
    "label": "导出透明背景的PNG",
    "click": function() {
      chart.exporting.exportPNG({
        "backgroundColor": "rgba(0, 0, 0, 0)" // 设置导出图片的背景色为透明
      });
    }
  }]
}];