borderDrawMode property
Border type of step area series.
Defaults to BorderDrawMode.top.
Also refer BorderDrawMode.
Widget build(BuildContext context) {
  return SfCartesianChart(
    series: <StepAreaSeries<SalesData, num>>[
      StepAreaSeries<SalesData, num>(
        borderDrawMode: BorderDrawMode.all,
      ),
    ],
  );
}
Implementation
final BorderDrawMode borderDrawMode;