Returns the tracker item for this chart.
        
        :return     <XChartTrackerItem> || None

def trackerItem( self ):
        """
        Returns the tracker item for this chart.
        
        :return     <XChartTrackerItem> || None
        """
        # check for the tracking enabled state
        if not self.isTrackingEnabled():
            return None
        
        # generate a new tracker item
        if not (self._trackerItem and self._trackerItem()):
            item = XChartTrackerItem()
            self.addItem(item)
            self._trackerItem = weakref.ref(item)
        
        return self._trackerItem()