Not able to draw text on top or arc in Flutter - dart

Question

I have drawn a custom arc using the draw arc method of the canvas library, now I want to write some text on top of this arc using TextPainter but when I paint the text on canvas I do not see anything. I just know the arc is drawn, and the text is invisible.
Is there some concept of Z-index or layering which I have to give to text paint to draw on top of the arc?
Below is my sample code
import 'dart:math' as math;
import 'package:flutter/material.dart';
class ChartPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
print(size.width);
print(size.height);
final Paint paint = new Paint()..color = Colors.blue;
// Draw a circle that circumscribes the arrow.
paint.style = PaintingStyle.stroke;
paint.strokeWidth = 50.0;
paint.isAntiAlias = true;
//canvas.drawCircle(new Offset(centerX, centerY), r, paint);
canvas.drawArc(new Rect.fromLTWH(0.0, 0.0, size.width, size.height), 0.0,
-math.pi / 4, false, paint);
paint.color = Colors.orange;
canvas.drawArc(new Rect.fromLTWH(0.0, 0.0, size.width, size.height),
-math.pi / 4, -math.pi / 4, false, paint);
paint.color = Colors.green;
canvas.drawArc(new Rect.fromLTWH(0.0, 0.0, size.width, size.height),
-math.pi / 2, -math.pi / 4, false, paint);
paint.color = Colors.purple;
canvas.drawArc(new Rect.fromLTWH(0.0, 0.0, size.width, size.height),
-math.pi * 3 / 4, -math.pi / 4, false, paint);
TextSpan span = new TextSpan(
style: new TextStyle(color: Colors.black, fontSize: 24.0,
fontFamily: 'Roboto'), text: "Ajay Singh");
TextPainter tp = new TextPainter(
text: span, textAlign: TextAlign.left);
tp.layout();
tp.paint(canvas, new Offset(100.0, 100.0));
}
#override
bool shouldRepaint(ChartPainter oldDelegate) {
return true;
}
}