At my app’s top screen I have this:

This is my code:

@override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    final center = Offset(size.width / 2, size.height / 2);
    return Row(
        crossAxisAlignment: CrossAxisAlignment.end,
        mainAxisSize: MainAxisSize.min,
        children: [
          RichText(
            text: TextSpan(
              children: <InlineSpan>[
                WidgetSpan(
                    child: ConstrainedBox(
                        constraints: const BoxConstraints(minWidth: 32),
                        child: const IntrinsicWidth(
                          child: TextField(
                              keyboardType: TextInputType.number,
                              maxLines: null,
                              decoration: InputDecoration(
                                  isDense: true,
                                  contentPadding: EdgeInsets.all(0))),
                        ))),
                const WidgetSpan(child: Text("xu00B3 + ")),
                WidgetSpan(
                    child: ConstrainedBox(
                        constraints: const BoxConstraints(minWidth: 32),
                        child: const IntrinsicWidth(
                          child: TextField(
                              keyboardType: TextInputType.number,
                              maxLines: null,
                              decoration: InputDecoration(
                                  isDense: true,
                                  contentPadding: EdgeInsets.all(0))),
                        ))),
                const WidgetSpan(child: Text("xu00B2 + ")),
                WidgetSpan(
                    child: ConstrainedBox(
                        constraints: const BoxConstraints(minWidth: 32),
                        child: const IntrinsicWidth(
                          child: TextField(
                              keyboardType: TextInputType.number,
                              maxLines: null,
                              decoration: InputDecoration(
                                  isDense: true,
                                  contentPadding: EdgeInsets.all(0))),
                        ))),
                const WidgetSpan(child: Text("x + ")),
                WidgetSpan(
                    child: ConstrainedBox(
                        constraints: const BoxConstraints(minWidth: 32),
                        child: IntrinsicWidth(
                          child: TextField(
                              keyboardType: TextInputType.number,
                              controller: controller,
                              maxLines: null,
                              decoration: const InputDecoration(
                                  isDense: true,
                                  contentPadding: EdgeInsets.all(0))),
                        ))),
              ],
            ),
          ),
          CustomPaint(
            foregroundPainter: LinePainter(_offsetStart, _offsetEnd),
            child: Container(
              padding: const EdgeInsets.all(0.0),
              height: 30.0,
              child: IconButton(
                  color: Colors.green,
                  onPressed: () {
                    setState(() {
                      _offsetStart = const Offset(10, 10);
                      _offsetEnd =
                          Offset(size.width, size.height);
                    });
                  },
                  icon: const Icon(Icons.done_outline_rounded),
                  splashColor: Colors.green,
                  tooltip: "Save equation"),
            ),
          ),
        ]);}

And LinePainter class:

class LinePainter extends CustomPainter {
  Paint _paint = Paint();
  Offset _offsetStart = const Offset(0, 0);
  Offset _offsetEnd = const Offset(60, 60);

  LinePainter(Offset offsetStart, Offset offsetEnd) {
    _offsetStart = offsetStart;
    _offsetEnd = offsetEnd;
    _paint = Paint()
      ..color = Colors.amber
      ..strokeWidth = 8.0;
  }

  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawLine(_offsetStart, _offsetEnd, _paint);
  }

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

(I was somewhat relying on this answer)

The problem is that when I draw a line it seems to start from the top of the container, but I need to draw a line starting from center of the entire phone screen.

Could somebody please help me achieve this? Thanks in advance.