Returns the items in this scene mapped with their dataset instance.
        
        :return     {<XChartDataset>: <QGraphicsItem>, ..}

def datasetItems(self):
        """
        Returns the items in this scene mapped with their dataset instance.
        
        :return     {<XChartDataset>: <QGraphicsItem>, ..}
        """
        out = {}
        for item in self.items():
            if isinstance(item, XChartDatasetItem):
                out[item.dataset()] = item
        return out