buildLegendItems method 
    
      
List<LegendItem>?
buildLegendItems(
 - int index, 
- LegendItemProvider provider
) 
    
    
  Implementation
  List<LegendItem>? buildLegendItems(int index, LegendItemProvider provider) {
  if (isVisibleInLegend) {
    _legendItem = ChartLegendItem(
      text: name ?? _defaultTrendlineName(),
      iconType:
          _toShapeMarkerType(legendIconType, legendItemProvider: provider),
      iconColor: color!,
      iconBorderWidth: 2,
      series: series,
      seriesIndex: series!.index,
      pointIndex: index,
      isToggled: isToggled,
      onTap: _handleLegendItemTapped,
      onRender: _handleLegendItemCreated,
    );
    return <LegendItem>[_legendItem!];
  } else {
    _legendItem = null;
    return null;
  }
}