custom graph overlay

main.dart

import 'package:flutter/material.dart';
import 'dart:ui' as ui;

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

class StartPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
          backgroundColor: Colors.white,
          body: SafeArea(
            child: CustomPaint(
              painter: PaintDemo(),
              size: Size.infinite,
            ),
          )
          /**/
          ),
    );
  }
}

class PaintDemo extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    // axis x
    canvas.drawLine(
        Offset(0, size.height / 2),
        Offset(size.width, size.height / 2),
        Paint()
          ..color = Colors.black
          ..strokeWidth = 1);

    // axis y
    canvas.drawLine(
        Offset(size.width / 2, 0),
        Offset(size.width / 2, size.height),
        Paint()
          ..color = Colors.black
          ..strokeWidth = 1);

    // top color
    canvas.saveLayer(
      Rect.fromLTRB(0, 0, size.width, size.height / 2),
      Paint()
        ..colorFilter = ColorFilter.mode(Colors.blue, BlendMode.srcATop)
        ..color = Colors.blue
        ..blendMode = BlendMode.srcATop,
    );

    _drawGraph(canvas, size);

    canvas.restore();

    // bottom color
    canvas.saveLayer(
      Rect.fromLTRB(0, size.height / 2, size.width, size.height),
      Paint()
        ..colorFilter = ColorFilter.mode(Colors.yellow, BlendMode.srcATop)
        ..color = Colors.yellow
        ..blendMode = BlendMode.srcATop,
    );

    _drawGraph(canvas, size);

    canvas.restore();
  }

  void _drawGraph(Canvas canvas, Size size) {
    canvas.drawLine(
        Offset(0, 0),
        Offset(size.width, size.height),
        Paint()
          ..isAntiAlias = true
          ..color = Colors.red
          ..style = PaintingStyle.stroke
          ..strokeWidth = 3);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}