docs: def updateTrackerItem( self, point = None ):
          """
          Updates the tracker item information.
          """
          item = self.trackerItem()
          if not item:
              return
          
          gridRect = self._buildData.get('grid_rect')
          
          if ( not (gridRect and gridRect.isValid()) ):
              item.setVisible(False)
              return
          
          if ( point is not None ):
              item.setPos(point.x(), gridRect.top())
          
          if ( not gridRect.contains(item.pos()) ):
              item.setVisible(False)
              return
          
          if ( self.chartType() != self.Type.Line ):
              item.setVisible(False)
              return
          
          if ( not self.isTrackingEnabled() ):
              item.setVisible(False)
              return
          
          item.rebuild(gridRect)