anchor.diff:  /// [LineChart] draws some [HorizontalLine] (set by [LineChartData.extraLinesData]),
   /// in below or above of everything, it draws from left to right side of the chart.
   class HorizontalLine extends FlLine {
  -
     /// Draws from left to right of the chart using the [y] value.
     final double y;
   
  
positive.code:     this.bar,
      this.barIndex,
      FlSpot spot,
      )   : spotIndex = bar.spots.indexOf(spot),
        super(spot.x, spot.y);
  }
  
  /// Holds data of showing each row item in the tooltip popup.
  class LineTooltipItem {
  
    /// Showing text.
    final String text;
  
  
negative.code: 
  /// Draws some straight horizontal or vertical lines in the [LineChart]
  class ExtraLinesData {
    final List<HorizontalLine> horizontalLines;
    final List<VerticalLine> verticalLines;
  
  