tooltipInfo method 
    
      
  
TooltipInfo?
tooltipInfo(
{ - Offset? position, 
- int? pointIndex, 
})override
     
    
    
  Implementation
  @override
TooltipInfo? tooltipInfo({Offset? position, int? pointIndex}) {
  final ChartPoint<D> point = ChartPoint<D>(
      x: series.circularXValues[currentSegmentIndex],
      y: series.circularYValues[currentSegmentIndex]);
  final Offset preferredPos = series.localToGlobal(calculateOffset(
      (_startAngle + _endAngle) / 2,
      (_innerRadius + _outerRadius) / 2,
      _center));
  return ChartTooltipInfo<T, D>(
    primaryPosition: preferredPos,
    secondaryPosition: preferredPos,
    text: series.tooltipText(point),
    header: '',
    data: series.dataSource![currentSegmentIndex],
    point: point,
    series: series.widget,
    renderer: series,
    seriesIndex: series.index,
    segmentIndex: currentSegmentIndex,
    pointIndex: currentSegmentIndex,
  );
}