Retrieving node locations from pydotplus (or any layered graph drawing engine)

Question

I'm preparing a layered graph drawing using a dataframe containing node data:

    type              label
0  Class  Insurance Product
1  Class             Person
2  Class            Address
3  Class   Insurance Policy

And another containing relationship data:

              froml                tol             rel fromcard tocard
0  Insurance Policy  Insurance Product  ConveysProduct      One    One
1            Person   Insurance Policy       hasPolicy      One   Many
2            Person            Address       ResidesAt     None   None

I populate a pydotplus dot graph with the content, which I can then use to generate a rendering:

pdp_graph = pydotplus.graphviz.Dot(graph_name="pdp_graph", graph_type='digraph', prog="dot")
for i,e in b_rels_df.iterrows():
    edge = pydotplus.graphviz.Edge(src=e['froml'], dst=e['tol'], label=e['rel'])#, set_fromcard=e['fromcard'], set_tocard=e['tocard'])
    pdp_graph.add_edge(edge)
    
for i,n in ents_df.iterrows():
    node = pydotplus.graphviz.Node(name=n['label'], set_type=n['type'], set_label=n['label'])
    pdp_graph.add_node(node)

png = pdp_graph.create_png()

display(Image(png))

So far so good - but now I want to retrieve the node positions for use in my own interactive layout (the png is a nice example/diagram, but I want to build upon it), so am attempting to retrieve the node locations calculated via:

[n.get_pos() for n in pdp_graph.get_nodes()]

But this only returns:

> [None, None, None, None]

I've tried lots of different methods, graphviz/dot are installed fine - as proven by the image of the layout - how can I extract the positions of the nodes as data from any type of dot-style layout?

There is a way I can do this via the pygraphviz library via networkx, but the installation-overhead restricts me (pygraphviz needs to be recompiled to cinch with the graphviz install) from being able to use that for the target installations where I've less control over the base environments, hence my attempt to use pydotplus, which appears less demanding in terms of install requirements.

How do I retrieve the layout data from a layered graph drawing using this setup (or one similar), such that I can use it elsewhere? I'm looking for x,y values that I can map back to the nodes that they belong to.

Answer 1

I know nothing about your python setup. ( As usual with python it seems awkward and restrictive )

I suggest using Graphviz directly. In particular the 'attributed DOT format' which is a plain text file containing the layout ( resulting from the layout engine ) produced when the engine is run with the command option -Tdot. The text file is easily parsed to get exactly what you need.

Here is a screenshot of the first paragraph of the relevant documentation

The graphviz.org website contains all the additional details you may need.

Answer 2

You are creating an output file of png format. All position data is lost in this process. Instead, create output format="dot". Then, read that back in & modify as desired.