borderWidth property
Border width of the interactive tooltip.
Used to change the stroke width of the axis tooltip.
Defaults to 0.
late TrackballBehavior trackballBehavior;
void initState() {
  trackballBehavior = TrackballBehavior(
    enable: true,
    tooltipSettings: InteractiveTooltip(
      borderColor: Colors.red,
      borderWidth: 3
    )
  );
  super.initState();
}
Widget build(BuildContext context) {
  return SfCartesianChart(
    trackballBehavior: trackballBehavior
  );
}
Implementation
final double borderWidth;