iPhone - refreshing UIView with overridden drawRect and viewDidAppear

Question

I have a written a tab-based application, which pulls data from an NSMutableArray, and graphs it in one view of a tab-based application. This works well the first time the view is viewed, but after this, the view does not refresh when it is viewed. I have overridden the viewDidLoad method in the viewController, but it does not seem to be working:

- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
[self.view setNeedsDisplay]; }

What is going wrong here? Is viewDidAppear supposed to be called each time the view is viewed, or have I misunderstood the method? Thanks for your efforts!

--James