borderWidth property
Border width of the marker.
Defaults to 2.
Widget build(BuildContext context) {
  return SfCartesianChart(
    series: <SplineSeries<SalesData, num>>[
      SplineSeries<SalesData, num>(
        markerSettings: MarkerSettings(
          isVisible: true,
          borderColor: Colors.red,
          borderWidth: 3
        ),
      ),
    ],
  );
}
Implementation
final double borderWidth;