def addExistingConnection(self, fromDagNode, toDagNode):
        fromDrawNode = self.drawNode(fromDagNode)
        toDrawNode = self.drawNode(toDagNode)
        if not fromDrawNode:
            raise RuntimeError("Attempting to connect node %s which is not yet registered to QGraphicsScene." % fromDagNode.name)
        if not toDrawNode:
            raise RuntimeError("Attempting to connect node %s which is not yet registered to QGraphicsScene." % toDagNode.name)
        newDrawEdge = DrawEdge(fromDrawNode, toDrawNode)
        self.addItem(newDrawEdge)
        return newDrawEdge
Adds a new draw edge for given from and to dag nodes.
dataset/ETHPy150Open andrew-gardner/dependsworkflow/depends/depends_graphics_widgets.py/SceneWidget.addExistingConnection