Syncs the scene together with the given datasets to make sure we
        have the proper number of items, by removing non-existant datasets
        and adding new ones.
        
        :param      scene    | <XChartScene>
                    datasets | <XChartDataset>
        
        :return     {<XChartDataset>: <XChartDatasetItem>, ..}

def calculateDatasetItems(self, scene, datasets):
        """
        Syncs the scene together with the given datasets to make sure we
        have the proper number of items, by removing non-existant datasets
        and adding new ones.
        
        :param      scene    | <XChartScene>
                    datasets | <XChartDataset>
        
        :return     {<XChartDataset>: <XChartDatasetItem>, ..}
        """
        dataitems = scene.datasetItems()
        olditems = set(dataitems.keys()).difference(datasets)
        newitems = set(datasets).difference(dataitems.keys())
        
        for dataset in olditems:
            scene.removeItem(dataitems[dataset])
        
        for dataset in newitems:
            dataitems[dataset] = scene.addDataset(dataset)
        
        return dataitems