How to enforce the left-to-right node ordering in GraphViz rank layout?

Question

I am visualizing a collection of process with GraphViz. Each process consists of some Read or Write operations in program order. Naturally, it is desirable to arrange the operations in the left-to-right order with respect to each process.

Using GraphViz (version 2.28), my code goes like this:

digraph G 
{
ranksep = 1.0; size = "10,10";
{ 
    node [shape = plaintext, fontsize = 20];
    0 -> 1 -> 2 -> 3 -> 4;
}
node [shape = box];
{rank = same;0;wy1;rf1;rc1;rz1;ry1;ra1;rb1;rx2;}
{rank = same;1;wf1;}
{rank = same;2;wx2;wc1;}
{rank = same;3;wf2;wz2;wx3;wa1;}
{rank = same;4;wz1;wy2;wx5;wb1;}
wy1 -> rf1;
rf1 -> rc1;
rc1 -> rz1;
rz1 -> ry1;
ry1 -> ra1;
ra1 -> rb1;
rb1 -> rx2;
wx2 -> wc1;
wf2 -> wz2;
wz2 -> wx3;
wx3 -> wa1;
wz1 -> wy2;
wy2 -> wx5;
wx5 -> wb1;
wf1 -> rf1[color = blue];
wc1 -> rc1[color = blue];
wz1 -> rz1[color = blue];
wy1 -> ry1[color = blue];
wa1 -> ra1[color = blue];
wb1 -> rb1[color = blue];
wx2 -> rx2[color = blue];

// W'WR Order:
wx3 -> wx2[style = dashed, color = red];

// W'WR Order:
wx5 -> wx2[style = dashed, color = red];
}

I am sorry to say that I am not allowed to post the output picture with too low reputation. If you can run the code, you will see that the result is not so satisfying due to the out of order in process with pid = 3. Specifically, GraphViz layout algorithm has rearranged the (ideal) order "wf2-> wz2 -> wa1 -> wx3" to "wx3, wf2, wz2, wa1". Therefore, my problem is:

My Problem: How to enforce the left-to-right node ordering in the rank environment?

With exploring in this site, I have found some similar problems and potential solutions. However, they just did not work in my specific example:

• Graphviz .dot node ordering: the constraint = false option made my PDF picture worse. I checked the dot User's Manual which says:

  During rank assignment, the head node of an edge is constrained to be on a higher rank than the tail node. If the edge has constraint=false, however, this requirement is not enforced.

  Based on the above statements, (I guess) constraint = false option takes effect between different ranks instead of in the same rank.

• Graphviz---random node order and edges going through labels: With surprise, the constraint = false option helped the "finite state machine" a lot in the same rank. Again, it does not save me from the trouble.

• graphviz: circular layout while preserving node order: The process graph is dynamic both in number of nodes and edges. So, it maybe not attractive to use the absolute position for nodes (to cause many edge crossing?).

  Thanks for any suggestions. And executable code will be appreciated very much.

Answer 1

    digraph G 
{
ranksep = 1.0; size = "10,10";
{ 
    node [shape = plaintext, fontsize = 20];
    0 -> 1 -> 2 -> 3 -> 4;
}
node [shape = box];
{
rank = same;
0->wy1->rf1->rc1->rz1->ry1->ra1->rb1->rx2 [color=white];
rankdir=LR;
}
{
rank = same;
1->wf1[color=white];
rankdir=LR
}
{
rank = same;
2->wx2->wc1[color=white];
rankdir=LR;
}
{
rank = same;
3->wf2->wz2->wx3->wa1[color=white];
rankdir=LR;
}
{
rank = same;
4->wz1->wy2->wx5->wb1[color=white];
rankdir=LR;
}
wy1 -> rf1;
rf1 -> rc1;
rc1 -> rz1;
rz1 -> ry1;
ry1 -> ra1;
ra1 -> rb1;
rb1 -> rx2;
wx2 -> wc1;
wf2 -> wz2;
wz2 -> wx3;
wx3 -> wa1;
wz1 -> wy2;
wy2 -> wx5;
wx5 -> wb1;
wf1 -> rf1[color = blue];
wc1 -> rc1[color = blue];
wz1 -> rz1[color = blue];
wy1 -> ry1[color = blue];
wa1 -> ra1[color = blue];
wb1 -> rb1[color = blue];
wx2 -> rx2[color = blue];

// W'WR Order:
wx3 -> wx2[style = dashed, color = red];

// W'WR Order:
wx5 -> wx2[style = dashed, color = red];
}

I am not quite sure that i've correctly got your problem, but try using this and comment please if it is what you want. I've added invisible edges for correct ranking of nodes and used rankdir to use left-right layout.