yValueMapper property Null safety

ChartIndexedValueMapper? yValueMapper

Field in the data source, which is considered as y-value.

Defaults to null

Widget build(BuildContext context) {
   return Container(
       child: SfCartesianChart(
           series: <ColumnSeries<SalesData, num>>[
               ColumnSeries<SalesData, num>(
                 dataSource: chartData,
                 xValueMapper: (SalesData sales, _) => sales.x,
                 yValueMapper: (SalesData sales, _) => sales.y,
final List<SalesData> chartData = <SalesData>[
   SalesData(1, 23),
   SalesData(2, 35),
   SalesData(3, 19)

class SalesData {
  SalesData(this.x, this.y);
  final double x;
  final double y;


final ChartIndexedValueMapper<dynamic>? yValueMapper;