tooltipInfo method 
    
      
  
TooltipInfo?
tooltipInfo(
{ - Offset? position, 
 
- int? pointIndex, 
 
})
      override
     
    
    
  Implementation
  @override
TooltipInfo? tooltipInfo({Offset? position, int? pointIndex}) {
  if (points.isEmpty) {
    return null;
  }
  final CartesianChartPoint<D> chartPoint = _chartPoint();
  pointIndex ??= currentSegmentIndex;
  final ChartMarker marker = series.markerAt(pointIndex);
  final double markerHeight =
      series.markerSettings.isVisible ? marker.height / 2 : 0;
  final Offset preferredPos = points[0];
  return ChartTooltipInfo<T, D>(
    primaryPosition:
        series.localToGlobal(preferredPos.translate(0, -markerHeight)),
    secondaryPosition:
        series.localToGlobal(preferredPos.translate(0, markerHeight)),
    text: series.tooltipText(chartPoint),
    header: series.parent!.tooltipBehavior!.shared
        ? series.tooltipHeaderText(chartPoint)
        : series.name,
    data: series.dataSource![pointIndex],
    point: chartPoint,
    series: series.widget,
    renderer: series,
    seriesIndex: series.index,
    segmentIndex: currentSegmentIndex,
    pointIndex: pointIndex,
    hasMultipleYValues: true,
    markerColors: <Color?>[fillPaint.color],
    markerType: marker.type,
  );
}