contains method
Implementation
@override
bool contains(DiagramPoint point) {
if (bounds.width == 0 || bounds.height == 0) return false;
final dx = (point.x - bounds.center.x) / (bounds.width / 2);
final dy = (point.y - bounds.center.y) / (bounds.height / 2);
return dx * dx + dy * dy <= 1;
}