Adds a new graphic to the scene.
        
        :param      typ | <str>
        
        :return     <XWalkthroughGraphic> || None

def addGraphic(self, typ='basic'):
        """
        Adds a new graphic to the scene.
        
        :param      typ | <str>
        
        :return     <XWalkthroughGraphic> || None
        """
        cls = XWalkthroughGraphic.find(typ)
        if not cls:
            return None

        graphic = cls()
        self.addItem(graphic)
        
        return graphic