circle method
Implementation
void circle(Canvas canvas, Rect rect, double value) {
final double opacity = (1.0 - (value / 4.0)).clamp(0.0, 1.0);
final Color colorWithOpacity = color.withOpacity(opacity);
final double size = rect.width / 2;
final double area = size * size;
final double radius = math.sqrt(area * value / 4);
final Paint paint = Paint()..color = colorWithOpacity;
canvas.drawCircle(rect.center, radius, paint);
}