docs: def centerOnItems(self, items = None):
          """
          Centers on the given items, if no items are supplied, then all
          items will be centered on.
          
          :param      items | [<QGraphicsItem>, ..]
          """
          if not items:
              rect = self.scene().visibleItemsBoundingRect()
              if not rect.width():
                  rect = self.scene().sceneRect()
              
              self.centerOn(rect.center())
          else:
              self.centerOn(self.scene().calculateBoundingRect(items).center())