paint method
Draw itself and its children, according to the calculated
box.offset
Implementation
@override
void paint(Context context) {
  super.paint(context);
  final vc = value.clamp(0.0, 1.0);
  final _valueColor = valueColor ?? PdfColors.blue;
  final _backgroundColor = backgroundColor ?? _valueColor.shade(0.1);
  if (vc < 1.0) {
    final epsilon = vc == 0 ? 0 : 0.01;
    context.canvas
      ..drawRect(box!.left + box!.width * vc - epsilon, box!.bottom,
          box!.width * (1 - vc) + epsilon, box!.height)
      ..setFillColor(_backgroundColor)
      ..fillPath();
  }
  if (vc > 0.0) {
    context.canvas
      ..drawRect(box!.left, box!.bottom, box!.width * vc, box!.height)
      ..setFillColor(_valueColor)
      ..fillPath();
  }
}