Should valueFormatter property use weak modifier?

Tihs property is found in `Charts-Swift.h` file, it's a delegate, should it use **weak** modifier to avoid retain cycle?
```
@property (nonatomic, strong) id _Nullable valueFormatter;
```

Start with the valueFormatter declaration in Charts-Swift.h and review the issue discussion about delegate ownership and retain cycles. Determine whether the property should use weak instead of strong, then verify the project still builds and related chart behavior remains intact.

Written by the indexing model from the issue text.