How do I order subgraph clusters when using dot?

Question

I have a dot file in which I create subgraph clusters which I want to appear in a specific order, let's say I have this:

digraph G {
    splines=true;
    sep="+25,25";
    overlap=scalexy;
    nodesep=0.6;
    subgraph cluster_2 {
        label="ADD_MORE_PROBLEMS";
        subgraph cluster_3 {
            label="pattern";
            N1 [label="problem"];
        }
        subgraph cluster_4 {
            label="replacement";
            N2 [label="problem"];
            N3 [label="problem"];
        }
    }
}

Which creates:

How do I ensure that "pattern" appears to the left of "replacement" (I may have an arbitrary number of subgraphs).

Answer 1

Clusters are one of the odd cases where simply the ordering in the code makes most (if not quite all) of the difference. If we simply reorder your code like this:

digraph G {
    splines=true;
    sep="+25,25";
    overlap=scalexy;
    nodesep=0.6;
    subgraph cluster_2 {
        label="ADD_MORE_PROBLEMS";
        subgraph cluster_4 {
            label="replacement";
            N2 [label="problem"];
            N3 [label="problem"];
        }
        subgraph cluster_3 {
            label="pattern";
            N1 [label="problem"];
        }
    }
}

that makes all the difference.

Now, that can fail, in which case setting up invisible edges is one of the more common solutions.

Answer 2

I can't give and answer, but can provide some clarification. The usual approach to force layout is to introduce hidden edges. In this case, it does not work.

Without the nested clusters, you can use rank=same to force connected edges onto the same level. Then, an invisible edge N1 -> N2 [style = invis] would force the nodes into the correct ordering.

However, constraining nodes with rank breaks the cluster membership and prevents the scheme from working.

The modified graph shows the result. There may not be a general solution.

digraph G {
    splines=true;
    sep="+25,25";
    overlap=scalexy;
    nodesep=0.6;
    subgraph cluster_2 {
        label="ADD_MORE_PROBLEMS";
        subgraph cluster_3 {
            label="pattern";
            N1 [label="problem 1"];
        }
        subgraph cluster_4 {
            label="replacement";
            N2 [label="problem 2"];
            N3 [label="problem 3"];
        }
        // Introduce hidden edge (shown dashed)
        N1 -> N2 [style = dashed];
        // Force nodes to remain at same rank
        { rank = same; N1; N2; }
    }
}