titleForBarAtIndex on Bar Graph not treated as Optional

the datasource delegate specifies 

```
@optional
- (UIColor *)colorForBarAtIndex:(NSInteger)index;
- (UIColor *)colorForBarBackgroundAtIndex:(NSInteger)index;
- (CFTimeInterval)animationDurationForBarAtIndex:(NSInteger)index;

- (NSString *)titleForBarAtIndex:(NSInteger)index;
```

however in it is being treated as required in the code.. 
