borderWidth property
Border width of the series.
Note: This is not applicable for line, spline, step line, and fast line series types.
Defaults to 0.
Widget build(BuildContext context) {
  return SfCartesianChart(
    series: <ColumnSeries<SalesData, String>>[
      ColumnSeries<SalesData, String>(
        borderColor: Colors.red,
        borderWidth: 2
      ),
    ],
  );
}
Implementation
final double borderWidth;