How to change origin of the custom paint in Flutter?

Just use the method translate in canvas: canvas.translate(0, size.height). But mind that in such a case, you will need to use negative values in the y axis.

If you want your canvas coordinates to behave like a classical graph, use the method scale:

  @override  void paint(Canvas canvas, Size size) {    canvas.translate(0, size.height);    canvas.scale(1, -1);    final paint = Paint();    paint.color = Colors.black;    canvas.drawLine(Offset.zero, Offset(500, 500), paint);  }

I am not really sure how can you manipulate the origin of Canvas area. You can, however, apply a translation on your Offset coordinates, which should allow you to place your line where you want eventually.

I have made this simple example, it may be of help:

import "package:flutter/material.dart";void main() {  runApp(new MaterialApp(home: new MyApp(),  ));}class MyApp extends StatefulWidget {  @override  _MyAppState createState() => new _MyAppState();}class _MyAppState extends State<MyApp> {  CustomPaint _myCustomPainter = new CustomPaint(    size: new Size(400.0, 400.0),    painter: new Line(),  );  @override  Widget build(BuildContext context) {    final key = new GlobalKey<ScaffoldState>();    return new Scaffold(      key: key,      body:      new GestureDetector(        onTap: () {          debugPrint("hello");        },        child:        new Container(            alignment: FractionalOffset.center,            child: _myCustomPainter        ),      ),);  }}class Line extends CustomPainter {  @override  void paint(Canvas canvas, Size size) {    // canvas.translate(0.0, 100.0);    canvas.drawLine(new Offset(100.0, -50.0).translate(0.0, 100.0),        new Offset(0.0, 100.0).translate(0.0, 100.0),        new Paint()    );  }  @override  bool shouldRepaint(Line oldDelegate) {    // Since this Line painter has no fields, it always paints    // the same thing, and therefore we return false here. If    // we had fields (set from the constructor) then we would    // return true if any of them differed from the same    // fields on the oldDelegate.    return false;  }}