This is the source code of the tutorial post: https://fun4code.com/custom-flutter-bar-chart/

main.dart

import 'package:flutter/material.dart';
import 'package:samples/progress_bar/progress_bar.dart';
import 'package:samples/withcher_chart/witcher_chart.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Column(
            children: <Widget>[
              const Text(
                'The Witcher bar chart',
                style: TextStyle(
                  fontFamily: 'thewitcher',
                  fontSize: 18,
                ),
              ),
              WitcherChart.withSampleData(),
            ],
          ),
        ),
      ),
    );
  }
}

witcher_chart.dart

import 'package:flutter/material.dart';
import 'package:charts_flutter/flutter.dart';

class BarData {
  final int coins;
  final int monster;

  const BarData(this.coins, this.monster);
}

class WitcherChart extends StatelessWidget {
  final List<Series<dynamic, String>> seriesList;

  const WitcherChart(this.seriesList, {Key? key}) : super(key: key);

  factory WitcherChart.withSampleData() => WitcherChart(_createSampleData());

  static final data = <BarData>[
    const BarData(1, 11),
    const BarData(2, 14),
    const BarData(3, 8),
    const BarData(4, 10),
    const BarData(5, 15),
    const BarData(6, 1),
    const BarData(7, 9),
    const BarData(8, 9),
    const BarData(8, 3),
    const BarData(10, 12),
    const BarData(11, 18),
    const BarData(12, 8),
    const BarData(13, 3),
    const BarData(14, 4),
    const BarData(15, 10),
    const BarData(16, 12),
    const BarData(17, 10),
    const BarData(18, 8),
    const BarData(19, 15),
    const BarData(20, 12),
  ];

  static List<Series<BarData, String>> _createSampleData() {
    final highestValue =
        data.reduce((curr, next) => curr.monster > next.monster ? curr : next);
    final smallestValue =
        data.reduce((curr, next) => curr.monster < next.monster ? curr : next);

    return [
      Series(
        id: 'Data',
        data: data,
        domainFn: (BarData data, _) => data.coins.toString(),
        measureFn: (BarData data, _) => data.monster,
        colorFn: (data, _) {
          if (data.monster == highestValue.monster) {
            return Color.fromHex(code: '#2d3561');
          }
          if (data.monster == smallestValue.monster) {
            return Color.fromHex(code: '#ff0000');
          }
          return Color.fromHex(code: '#af0404');
        },
      ),
    ];
  }

  static final yStaticLabels = <TickSpec<int>>[
    // Possible to define style of labels directly inside
    const TickSpec(
      5,
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
      ),
    ),
    const TickSpec(
      10,
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
      ),
    ),
    const TickSpec(
      15,
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
      ),
    ),
    TickSpec(
      20,
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
        color: ColorUtil.fromDartColor(Colors.greenAccent),
      ),
    ),
  ];

  static final xStaticLabels = <TickSpec<String>>[
    // Possible to define style of labels directly inside
    const TickSpec(
      "1",
      label: "1 monster",
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
      ),
    ),
    const TickSpec(
      "10",
      label: "10 monsters",
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
      ),
    ),
    TickSpec(
      "20",
      label: "20 monsters",
      style: TextStyleSpec(
        fontFamily: 'thewitcher',
        color: ColorUtil.fromDartColor(Colors.orangeAccent),
      ),
    ),
  ];

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(15.0),
      child: SizedBox(
        height: 200,
        child: BarChart(
          seriesList,
          animate: true,
          defaultRenderer: BarRendererConfig(
            cornerStrategy: const ConstCornerStrategy(7),
          ),
          primaryMeasureAxis: NumericAxisSpec(
            tickFormatterSpec: BasicNumericTickFormatterSpec(
                (num? value) => "$value \u{1F4B0}"),
            tickProviderSpec: StaticNumericTickProviderSpec(yStaticLabels),
            renderSpec: GridlineRendererSpec(
              labelStyle: TextStyleSpec(
                fontSize: 14,
                color: Color.fromHex(code: '#f8b739'),
              ),
              labelAnchor: TickLabelAnchor.after,
            ),
          ),
          domainAxis: OrdinalAxisSpec(
            tickProviderSpec: StaticOrdinalTickProviderSpec(xStaticLabels),
            renderSpec: SmallTickRendererSpec(
              labelStyle: TextStyleSpec(
                lineHeight: 3,
                fontSize: 12,
                color: Color.fromHex(code: '#4d80e4'),
              ),
              lineStyle: const LineStyleSpec(
                color: Color.transparent,
              ),
            ),
          ),
        ),
      ),
    );
  }
}