start property
Start value of the multi-level label. The value from where the multi-level label border needs to start.
The start value for the category axis needs to string type, in the case
of date-time or date-time category axes need to be date-time and in the
case of numeric or logarithmic axes need to double.
Defaults to 'null'
Widget build(BuildContext context) {
  return Container(
    child: SfCartesianChart(
      primaryXAxis:
        NumericAxis(multiLevelLabels: const <NumericMultiLevelLabel>[
          NumericMultiLevelLabel(
            start: 0,
            end: 4,
            text: 'First',
          )
        ]
      )
    )
  );
}
Implementation
final T start;