numberToString method Null safety
override
    String describing just the numeric value without a unit suffix.
Implementation
@override
String numberToString() {
  final double? v = value;
  if (v == null) {
    return value.toString();
  }
  return '${(clampDouble(v, 0.0, 1.0) * 100.0).toStringAsFixed(1)}%';
}