solo method
Hide everything except color. Useful for "solo" legend interactions
(double-tap or long-press).
Implementation
void solo(Color color, Iterable<Color> all) {
final target = {for (final c in all) if (c != color) c};
if (!setEquals(target, _hidden)) {
_hidden
..clear()
..addAll(target);
notifyListeners();
}
}