A mathematical graph sample code with CustomPainter and Canvas.

math_graph.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
// Based on https://youtu.be/vvI_NUXK00s
    return FittedBox(
      fit: BoxFit.fill,
      child: SizedBox(
        width: 250,
        height: 150,
        child: Container(
          color: Colors.grey[200],
          child: CustomPaint(
            painter: PathPainter(),
          ),
        ),
      ),
    );
  }
}

class PathPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Offset pointA = Offset(size.width * 1 / 16, size.height * 3 / 4);
    Offset pointB = Offset(size.width * 11 / 16, size.height * 1 / 4);

    double handleW = size.width * 2 / 64;
    Rect handleA =
        Rect.fromCenter(center: pointA, width: handleW, height: handleW);
    Rect handleB =
        Rect.fromCenter(center: pointB, width: handleW, height: handleW);

    Paint paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 2.0;

    Path axes = Path()
      ..moveTo(size.width / 2, 0)
      ..lineTo(size.width / 2, size.height)
      ..moveTo(0, size.height * 3 / 5)
      ..lineTo(size.width, size.height * 3 / 5)
      ..close();

    Path line = Path()
      ..moveTo(pointA.dx, pointA.dy)
      ..lineTo(pointB.dx, pointB.dy)
      ..close();

    Path handles = Path()
      ..addRect(handleA)
      ..addRect(handleB)
      ..close();

    canvas.drawPath(axes, paint..color = Colors.black);
    canvas.drawPath(line, paint..color = Colors.red);
    paint.strokeWidth = 1.0;
    canvas.drawPath(handles, paint..color = Colors.black);
  }

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