borderWidth property
Specifies the border width of multi-level labels.
Defaults to '0'
Widget build(BuildContext context) {
  return Container(
    child: SfCartesianChart(
      primaryXAxis: NumericAxis(
        multiLevelLabelStyle: MultiLevelLabelStyle(
          borderWidth: 2.0
        ),
        multiLevelLabels: const <NumericMultiLevelLabel>[
          NumericMultiLevelLabel(
            start: 0,
            end: 4,
            text: 'First',
          )
        ]
      )
    )
  );
}
Implementation
final double borderWidth;