minorTickLines property
Customizes the appearance of the minor tick lines.
Minor ticks are small lines used to indicate the minor intervals between a major interval.
Widget build(BuildContext context) {
    return Container(
        child: SfCartesianChart(
           primaryXAxis:
             NumericAxis(minorTickLines: MinorTickLines(width: 2)),
        )
    );
}
Implementation
final MinorTickLines minorTickLines;