Forcefully draw edges on top of each other

4

[NOTE: I am not a Graphviz developer, just a “friend of the family”. I don’t speak for the developers]
Cool, I think you found a fascinating “accidental” result of a complex set of algorithms. Not documented that I know of, but not necessarily a bug. A gray area.
But can you depend on it, probably, but let me suggest a “more” legal alternative.
The basic idea is :

  1. you define the complete graph excluding all the “on-top” edges
  2. run it through dot with this command: dot -Tdot myfile.gv >myfile.dot
  3. you modify (edit) myfile.dot duplicating the “bottom” edges to create the “top” edges
  • change attribute values (color, penwidth, style, …) as needed
  • this process can be done manually (see below), with any programming language, or with the Graphviz GVPR language
    Then run the edited myfile.dot through this command: neato -n2 -Tpng myfie.dot >myfile.png (note the -n2 option)
    The FAQ has a section that helps explain, as does the documentation on dot as an output file format
    Here is an example of the result of step 3:
    graph {
    graph [bb=“0,0,252,108”];
    node [label="\N"];
    A [height=0.5,
    pos=“27,90”,
    width=0.75];
    X [height=0.5,
    pos=“81,18”,
    width=0.75];
    A:e – X:w [color="#000000",
    penwidth=9.0,
    pos=“55,90 71.006,90 36.994,18 53,18”];
    // duplicated by hand
    A:e – X:w [color="#ff00ff",
    penwidth=3.0,
    pos=“55,90 71.006,90 36.994,18 53,18”];
    B [height=0.5,
    pos=“99,90”,
    width=0.75];
    Y [height=0.5,
    pos=“153,18”,
    width=0.75];
    B:e – Y:w [color="#000000",
    penwidth=9.0,
    pos=“127,90 143.01,90 108.99,18 125,18”];
    // again
    B:e – Y:w [color="#ffff00",
    penwidth=3.0,
    pos=“127,90 143.01,90 108.99,18 125,18”];
    C [height=0.5,
    pos=“171,90”,
    width=0.75];
    Z [height=0.5,
    pos=“225,18”,
    width=0.75];
    C:e – Z:w [color="#000000",
    penwidth=9.0,
    pos=“199,90 215.01,90 180.99,18 197,18”];
    // number 3
    C:e – Z:w [color="#00ffff",
    penwidth=3.0,
    pos=“199,90 215.01,90 180.99,18 197,18”];
    }

Convoluted, yes. But definitely within the bounds of legal.
If you decide to try this, you could add “extra” attributes to the edges you wanted to duplicate like dup_pen=3 and dupcolor=blue (documented as legal). to give instructions to your script.

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