header property
Header of the tooltip. By default, the series name will be displayed in the header.
late TooltipBehavior tooltipBehavior;
void initState() {
  tooltipBehavior = TooltipBehavior(
    enable: true,
    header: 'Default'
  );
  super.initState();
}
Widget build(BuildContext context) {
  return SfCartesianChart(
    tooltipBehavior: tooltipBehavior
  );
}
Implementation
final String? header;