Bar, BarTween, BarPainter. Height property only.

bar.dart

import 'dart:ui' show lerpDouble;

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

class Bar {
  Bar(this.height);

  final double height;

  static Bar lerp(Bar begin, Bar end, double t) {
    return Bar(lerpDouble(begin.height, end.height, t));
  }
}

class BarTween extends Tween<Bar> {
  BarTween(Bar begin, Bar end) : super(begin: begin, end: end);

  @override
  Bar lerp(double t) => Bar.lerp(begin, end, t);
}

class BarChartPainter extends CustomPainter {
  static const barWidth = 10.0;

  BarChartPainter(Animation<Bar> animation)
      : animation = animation,
        super(repaint: animation);

  final Animation<Bar> animation;

  @override
  void paint(Canvas canvas, Size size) {
    final bar = animation.value;
    final paint = Paint()
      ..color = Colors.blue[400]
      ..style = PaintingStyle.fill;
    canvas.drawRect(
      Rect.fromLTWH(
        (size.width - barWidth) / 2.0,
        size.height - bar.height,
        barWidth,
        bar.height,
      ),
      paint,
    );
  }

  @override
  bool shouldRepaint(BarChartPainter old) => false;
}