BarChart -> show persist tooltip, and change color when click happens on bars.

main.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'FlChart Demo',
      showPerformanceOverlay: false,
      theme: ThemeData(
        primaryColor: const Color(0xff262545),
        primaryColorDark: const Color(0xff201f39),
        brightness: Brightness.dark,
      ),
      home: Scaffold(
        backgroundColor: Color(0xffdddddd),
        appBar: AppBar(
          title: Text("fl_chart"),
        ),
        body: MyBarChart(),
      ),
    );
  }
}

class MyBarChart extends StatefulWidget {
  @override
  _MyBarChartState createState() => _MyBarChartState();
}

class _MyBarChartState extends State<MyBarChart> {
  Color touchedColor;
  Color defaultColor;

  int selectedBarIndex;

  List<double> values = [4, 2, 5, 8, 2];

  List<int> touchedIndices = [];

  int lastTouchStartOnIndex = -1;

  @override
  void initState() {
    super.initState();
    touchedColor = Colors.blueAccent;
    defaultColor = touchedColor.withOpacity(0.4);
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Padding(
        padding: EdgeInsets.only(right: 40),
        child: AspectRatio(
          aspectRatio: 2,
          child: BarChart(
            BarChartData(
              alignment: BarChartAlignment.spaceEvenly,
              barGroups: values.asMap().entries.map((MapEntry<int, double> entry) {
                final xValue = entry.key;
                final yValue = entry.value;
                final touched = touchedIndices.contains(xValue);
                return BarChartGroupData(
                    x: xValue,
                    barRods: [
                      BarChartRodData(
                        y: yValue,
                        colors: [
                          touched ? touchedColor : defaultColor,
                        ],
                      )
                    ],
                    showingTooltipIndicators: touched ? [0] : []);
              }).toList(),
              maxY: 10,
              barTouchData: BarTouchData(
                handleBuiltInTouches: false,
                touchCallback: (barTouchResponse) {
                  if (barTouchResponse.touchInput is PointerDownEvent &&
                      barTouchResponse.spot != null) {
                    lastTouchStartOnIndex = barTouchResponse.spot.touchedBarGroupIndex;
                  } else if (barTouchResponse.touchInput is PointerUpEvent) {
                    final PointerUpEvent PointerUpEven = barTouchResponse.touchInput;
                    // Tap happened
                    setState(() {
                      if (touchedIndices.contains(lastTouchStartOnIndex)) {
                        touchedIndices.remove(lastTouchStartOnIndex);
                      } else {
                        touchedIndices.add(lastTouchStartOnIndex);
                      }
                      lastTouchStartOnIndex = -1;
                    });
                  }
                },
              ),
            ),
          ),
        ),
      ),
    );
  }
}