Basic bar chart

main.dart

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

void main() {
  runApp(BarChartDemo());
}

class BarChartDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text("Bar chart demo"),
        ),
        body: Padding(
          padding: const EdgeInsets.all(8.0),
          child: BarChart(),
        ),
      ),
    );
  }
}

class BarChart extends StatelessWidget {
  final Map<String, int> data = {
    "Banana": 16,
    "Orange": 8,
    "Apple": 10,
    "Kiwi": 10,
    "Pear": 3
  };

  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: BarChartPainter(data, "Favorite Fruit"),
      child: Container(
        width: 350,
      ),
    );
  }
}

class BarChartPainter extends CustomPainter {
  final String title;
  final Map<String, int> data;
  // Currently based on the length of the category names
  double marginTopX = 0;
  double marginTopY;
  //padding between the bars
  final double paddingY = 5;
  final double axisWidth = 2;
  final double barHeight = 15;
  BarChartPainter(this.data, this.title) {
    // determine where to begin with X, based on the width of the category names
    data.forEach((key, value) {
      var text = createText(key, 1);
      if ((text.width + 5) > marginTopX) {
        marginTopX = text.width + 5;
      }
    });
    marginTopY = createText(title, 1.5).height + paddingY;
  }
  @override
  void paint(Canvas canvas, Size size) {
    Paint axis = Paint()
      ..strokeWidth = axisWidth
      ..color = Colors.grey;

    double number = 0;
    data.forEach((key, value) {
      drawBar(canvas, size, number, key, value);
      number++;
    });
    drawAxes(canvas, size, axis);
    drawTitle(canvas, size);
  }

  void drawTitle(Canvas canvas, Size size) {
    TextPainter tp = createText(title, 1.5);
    tp.paint(canvas, new Offset(size.width / 2 - tp.width / 2, 0));
  }

  void drawAxes(Canvas canvas, Size size, Paint axis) {
    canvas.drawLine(
      Offset(marginTopX,
          data.entries.length * (paddingY + barHeight) + marginTopY),
      Offset(size.width,
          data.entries.length * (paddingY + barHeight) + marginTopY),
      axis,
    );
    canvas.drawLine(
      Offset(marginTopX,
          data.entries.length * (paddingY + barHeight) + marginTopY),
      Offset(marginTopX, marginTopY - paddingY),
      axis,
    );
  }

  void drawBar(Canvas canvas, Size size, double number, String key, int value) {
    double y = number * (paddingY + barHeight) + marginTopY + barHeight / 2;
    drawText(key, canvas, y);
    Paint paint = Paint()
      ..strokeWidth = barHeight
      ..color = Colors.green;
    canvas.drawLine(
      Offset(marginTopX, y),
      Offset((size.width - marginTopX) / (maxValue() / value) + marginTopX, y),
      paint,
    );
  }

  void drawText(String key, Canvas canvas, double y) {
    TextPainter tp = createText(key, 1);
    tp.paint(canvas, new Offset(0, y - tp.height / 2));
  }

  TextPainter createText(String key, double scale) {
    TextSpan span =
        new TextSpan(style: new TextStyle(color: Colors.grey[600]), text: key);
    TextPainter tp = new TextPainter(
        text: span,
        textAlign: TextAlign.left,
        textScaleFactor: scale,
        textDirection: TextDirection.ltr);
    tp.layout();
    return tp;
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => false;

  int maxValue() => data.values.reduce(max);
}