最终效果如下：
在这里插入图片描述

绘制

先创建一个CustomPainter，使用canvas绘制。

import 'dart:ui';
import 'package:flutter/material.dart';
/// 自定义折线图
class LineChartPainter extends CustomPainter { 
  Paint _outlinePaint;
  Paint _axisXPaint;
  Paint _valuePaint;
  /// 左边Y轴标签
  TextPainter _leftLabelPainter;
  List<int> _yTitle = [40, 30, 20, 10, 0, -10]; // y轴刻度数量
  List<double> _yData;
  final int averageY = 5; // y轴平均份数
  final int gapY = 10; // y轴平均间隔值
  final double _yTitlePadding = 5;
  LineChartPainter(this._yData, { Listenable repaint})
      : super(repaint: repaint) { 
    _outlinePaint = Paint()
      ..color = Colors.grey
      ..style = PaintingStyle.stroke
      ..strokeWidth = 1;
    _axisXPaint = Paint()
      ..color = Colors.grey
      ..style = PaintingStyle.stroke
      ..strokeWidth = 1;
    _valuePaint = Paint()
      ..color = Colors.blue
      ..style = PaintingStyle.stroke
      ..strokeWidth = 1.5;
    _leftLabelPainter = TextPainter()..textDirection = TextDirection.ltr;
  }
  @override
  void paint(Canvas canvas, Size size) { 
    print("${this},,,$size");
    /// 外边框矩形
    canvas.drawRect(
        Rect.fromLTRB(0, 0, size.width, size.height), _outlinePaint);
    /// x轴
    for (var i = 0; i < 4; i++) { 
      canvas.drawLine(Offset(0, size.height / averageY * (i + 1)),
          Offset(size.width, size.height / averageY * (i + 1)), _axisXPaint);
    }
    /// y轴标题
    for (var i = 0; i <= 5; i++) { 
      _leftLabelPainter.text = TextSpan(
          text: _yTitle[i].toString(), style: TextStyle(color: Colors.black));
      _leftLabelPainter.layout();
      _leftLabelPainter.paint(
        canvas,
        Offset(-_leftLabelPainter.width - _yTitlePadding,
            size.height / 5 * i - _leftLabelPainter.height / 2),
      );
    }
    /// 数据
    /// 需要将y轴数据转换为height中对应比例的高度。
    var points = <Offset>[];
    for (var i = 0; i < _yData.length; i++) { 
      points.add(Offset(size.width / _yData.length * i,
          translateValue(size.height, _yData[i])));
    }
    canvas.drawPoints(PointMode.polygon, points, _valuePaint);
  }
  @override
  bool shouldRepaint(LineChartPainter oldDelegate) { 
    return oldDelegate._yData != _yData;
  }
  /// 转换y坐标
  double translateValue(double height, double rawValue) { 
    /// y轴真实值总长度
    var valueSum = averageY * gapY;
    /// 真实值和height计算比例
    var scale = height / valueSum;
    var result = rawValue * scale;
    return height - result - height / averageY;
  }
}

实现过渡动画

使用CustomPaint包裹CustomPainter，为了在数据变化时，有动画效果，使用ImplicitlyAnimatedWidget。
实现动画效果的重点是forEachTween和lerp方法。
注意：State不要继承ImplicitlyAnimatedWidgetState，直接继承AnimatedWidgetBaseState即可，它内部实现了对AnimationController的监听，并实时刷新Animation的value,自动调用forEachTween方法。

import 'dart:ui';
import 'package:flutter/material.dart';
import 'line_chart_painter.dart';
/// 包装折线图 数据更新时展示过渡动画
class LineChart extends ImplicitlyAnimatedWidget { 
  final Size size;
  final LineChartData lineChartData;
  final Duration duration;
  const LineChart(
      { Key key,
      this.size,
      @required this.lineChartData,
      @required this.duration})
      : super(key: key, duration: duration);
  @override
  _CustomCanvasState createState() { 
    return _CustomCanvasState();
  }
}
class _CustomCanvasState extends AnimatedWidgetBaseState<LineChart> { 
  DataTween _dataTween;
  @override
  Widget build(BuildContext context) { 
    return LayoutBuilder(
      builder: (context, constraint) { 
        print(
            "widget size:${widget.size} ${constraint.maxWidth}...${constraint.maxHeight}");
        var constrainSize =
            widget.size ?? Size(constraint.maxWidth, constraint.maxHeight);
        if (constrainSize.width == double.infinity) { 
          throw FlutterError("必须为组件或父widget设置一个有效宽度");
        }
        if (constrainSize.height == double.infinity) { 
          throw FlutterError("必须为组件或父widget设置一个有效高度");
        }
        return CustomPaint(
          foregroundPainter: LineChartPainter(
              _dataTween.evaluate(animation).data,
              repaint: animation),
          size: constrainSize,
        );
      },
    );
  }
  @override
  void forEachTween(TweenVisitor visitor) { 
    /// 第一个参数是初始的tween,第二个参数是目标值，第三个是生成tween的回调。
    _dataTween = visitor(
        _dataTween, widget.lineChartData, (value) => DataTween(begin: value));
  }
}
class DataTween extends Tween<LineChartData> { 
  DataTween({ LineChartData begin, LineChartData end})
      : super(begin: begin, end: end);
  @override
  LineChartData lerp(double t) => LineChartData.lerp(begin, end, t);
}
class LineChartData { 
  List<double> data;
  LineChartData({ this.data});
  /// 计算动画更新时的数据
  /// begin表示动画开始时的数据，end是结束时的数据，t是动画估值器，从0到1，代表动画运行的进度。
  static LineChartData lerp(LineChartData begin, LineChartData end, double t) { 
    /// 根据begin和end每个对应的值，因为值类型是double,所以使用系统自带的lerpDouble来计算值。
    LineChartData result;
    if (begin.data != null &&
        end.data != null &&
        begin.data.length == end.data.length) { 
      result = LineChartData(
          data: List.generate(begin.data.length, (index) { 
        return lerpDouble(begin.data[index], end.data[index], t);
      }));
    } else if (begin.data.length > end.data.length) { 
      result = LineChartData(
          data: List.generate(end.data.length, (index) { 
        return lerpDouble(begin.data[index], end.data[index], t);
      }));
    } else if (begin.data.length < end.data.length) { 
      result = LineChartData(
          data: List.generate(begin.data.length, (index) { 
        return lerpDouble(begin.data[index], end.data[index], t);
      }));
    }
    return result;
  }
}

使用

class StudyCanvasPage extends StatefulWidget { 
  const StudyCanvasPage({ Key key}) : super(key: key);
  @override
  _StudyCanvasPageState createState() => _StudyCanvasPageState();
}
class _StudyCanvasPageState extends State<StudyCanvasPage> { 
  var check = false;
  List<double> _yData = [29.2, 29.8, 29.3, 29.2, 29.4, 29.1, 29.3, 29.2, 29.5, 29.2, 29.2, 29, 29.3, 29.2, 29.4, 29.1, 29.3, 29.2, 29.5, 29.2];
  List<double> _yData1 = [12.2, 19.8, 19.3, 9.2, 19.4, 19.1, 19.3, 19.2, 19.5, 19.2, 19.2, 19, 19.3, 19.2, 19.4, 19.1, 19.3, 19.2, 19.5, 19.2,];
  @override
  Widget build(BuildContext context) { 
    return Scaffold(
      appBar: AppBar(
        title: Text('study canvas'),
      ),
      body: SafeArea(
        child: Container(
          child: Column(
            children: [
              ElevatedButton(
                onPressed: () { 
                  setState(() { 
                    check = !check;
                  });
                },
                child: Text("切换数据"),
              ),
              Expanded(
                child: Padding(
                  padding: const EdgeInsets.all(28.0),
                  child: LineChart(
                    lineChartData:
                        LineChartData(data: check ? _yData : _yData1),
                    duration: Duration(milliseconds: 300),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}