class_docstr: A DummyVertex is used for edges that span over several layers, it's inserted in every inner layer.
  
  **Attributes**
      - view (viewclass): since a DummyVertex is acting as a Vertex, it must have a view.
      - ctrl (list[_sugiyama_attr]): the list of associated dummy vertices.
code: def __str__(self):
      s = "(%3d,%3d) x=%s" % (self.layer, self.pos, str(self.x))
      if self.dummy:
          s = "[d] %s" % s
      return s
  
file_path: pygraphistry/graphistry/layout/utils/dummyVertex.py
imports: from .rectangle import Rectangle
  from .layoutVertex import LayoutVertex
label_desc: Use image node_id 4 for calling the DummyVertex obj's underlying member method code with example usage: obj.__str__() and returns: s