anchor.diff:    static final String line = 'line';
   }
   
  -abstract class Chart<T extends ChartModel> {
  +abstract class Chart<S extends Comparable, C extends Comparable, T extends ChartModel<S, C>> {
   
     Element get element;
   
  
positive.code:   }
  }
  
  class _AreaChartImpl extends _BaseChartImpl<CategoryModel> implements AreaChart {
  
    _AreaChartImpl({String titleText, String subtitleText,
      dynamic width, dynamic height, ChartXAxis xAxis, ChartYAxis yAxis}):
    super(ChartType.area, titleText: titleText, subtitleText: subtitleText,
      width: width, height: height, xAxis: xAxis, yAxis: yAxis);
  
    List<ChartDataSets> get series {
      final list = <ChartDataSets>[];
      final data = <String, List<int>>{};
  
      for (List entry in model.keys) {
        final series = entry[0], category = entry[1];
  
negative.code:   }
  }
  
  class _ColumnChartImpl<S extends Comparable, C extends Comparable> extends _BaseChartImpl<S, C, CategoryModel<S, C>> implements ColumnChart<S, C> {
  
    _ColumnChartImpl({String titleText, String subtitleText,
      dynamic width, dynamic height, ChartXAxis xAxis, ChartYAxis yAxis}):
    super(ChartType.column, titleText: titleText, subtitleText: subtitleText,
      width: width, height: height, xAxis: xAxis, yAxis: yAxis);
  
    @override
    List<ChartDataSets> get series {
      final list = <ChartDataSets>[];
      final data = <S, List<int>>{};
  
      for (List entry in model.keys) {
        final series = entry[0], category = entry[1];
  