Cell of UICollectionView not always refreshed after a scroll

I have defined in my Storyboard (iPad) a view that contains a UICollectionView, at the bottom of the view there's also a UIToolbar. In the UICollectionView I have added a UICollectionViewCell (implemented by an iPadCellCollectionViewCell class) that contains a another view that is a Core-Plot Graph (a CPTGraphHostingView class).

I have a class called X that implements the UICollectionViewDelegate and UICollectionViewDataSource.

In the class X, I build for each cell of my view (in ViewDidLoad) the Core-Plot Graph and I have the following code to link the graph to the cell of the UICollectionViewCell

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
static NSString* cellId = @"ChartCollectionId"; 

iPadCellCollectionViewCell* cell = [self.theCollectionView dequeueReusableCellWithReuseIdentifier:cellId forIndexPath:indexPath];

MyChart* chart = [_charts objectAtIndex:indexPath.row];

cell.hostingView.hostedGraph = chart.barChart;

return cell;

}

It's working fine, all my Graphs are correctly displayed on the screen. The issue happens when the user scroll the view, then one or several cells become "blank". I really don't understand why.

I have found a workaround, instead of adding the CPTGraphHostingView in the UICollectionViewCell in the Interface Builder, I build it myself and the previous code becomes:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
static NSString* cellId = @"ChartCollectionId"; 

iPadCellCollectionViewCell* cell = [self.theCollectionView dequeueReusableCellWithReuseIdentifier:cellId forIndexPath:indexPath];


[cell.contentView addSubview:[_hostingViews objectAt