selectedBorderWidth property

Border width of the selected data points or series.

late SelectionBehavior selectionBehavior;

void initState() {
  selectionBehavior = SelectionBehavior(
    enable: true,
    selectedBorderWidth: 4,
    selectedBorderColor: Colors.red
  );
  super.initState();
}

Widget build(BuildContext context) {
  return SfCartesianChart(
    series: <BarSeries<SalesData, num>>[
      BarSeries<SalesData, num>(
        selectionBehavior: selectionBehavior
      ),
    ],
  );
}