Dot language with concentrate=true confusing the graph

Question

I draw a simple graph as shown below and found different behavior of the color display.

digraph G {
     concentrate=true
    edge [dir=none]
     a  -> b [color=red]
     b -> c   
     c -> b 
     b -> a 
    }

Graph show edge between a and b is red >> this one is correct.

But when I just change it to be

     digraph G {
     concentrate=true
    edge [dir=none]
     a  -> b 
     b -> c   
     c -> b 
     b -> a [color=red]
    }

This time the color of edge a and b is black not red color as I want. Could someone figure out what I do wrong here?

Answer 1

You can use ports to prevent edges from overlap:

digraph G {
  concentrate=true
  edge [dir=none]
  a  -> b 
  b -> c   
  c -> b 
  b:nw -> a:se [color=red]
}

See also: https://graphviz.gitlab.io/_pages/doc/info/lang.html

Answer 2

Analysis

Because the code in the question draws two edges one on top of the other, the color of the last edge drawn wins. The layout engine for Dot works either Bottom to Top or Right to Left. This means that the last edge drawn is the first one listed.

General Solution

Dot draws Digraphs. An undirected graph can be simulated using dir=none but then there should only be one edge:

digraph G {
  concentrate=true
  edge [dir=none]
  a -> b [color=red]
  b -> c
 }

Alternative

Keep in mind that dir=none is not primarily intended as a display property. If the goal is two directed edges, then dir=both is a better alternative:

digraph G {
  concentrate=true
  edge [dir=both]
  a -> b [color="red:black"]
  b -> c [color="black:black]
}

Notes

It is useful to conceptually separate modeling the graph from the display properties. Dot doesn't make this particularly easy because it encourages inlining style information. But the price of separating concerns probably pays off in debugging time.