borderWidth property
Border width of the data label.
Defaults to 0.
Widget build(BuildContext context) {
  return SfCartesianChart(
    series: <SplineSeries<SalesData, num>>[
      SplineSeries<SalesData, num>(
        dataLabelSettings: DataLabelSettings(
          isVisible: true,
          borderColor: Colors.red,
          borderWidth: 2
        ),
      ),
    ],
  );
}
Implementation
final double borderWidth;