Overloads the moving event to move the tracker item.
        
        :param      event | <QEvent>

def mouseMoveEvent( self, event ):
        """
        Overloads the moving event to move the tracker item.
        
        :param      event | <QEvent>
        """
        super(XChartScene, self).mouseMoveEvent(event)
        
        self.updateTrackerItem(event.scenePos())