GraphViz - How to connect subgraphs?

191
votes

In the DOT language for GraphViz, I'm trying to represent a dependency diagram. I need to be able to have nodes inside a container and to be able to make nodes and/or containers dependent on other nodes and/or containers.

I'm using subgraph to represent my containers. Node linking works just fine, but I can't figure out how to connect subgraphs.

Given the program below, I need to be able to connect cluster_1 and cluster_2 with an arrow, but anything I've tried creates new nodes instead of connecting the clusters:

digraph G {

    graph [fontsize=10 fontname="Verdana"];
    node [shape=record fontsize=10 fontname="Verdana"];

    subgraph cluster_0 {
        node [style=filled];
        "Item 1" "Item 2";
        label = "Container A";
        color=blue;
    }

    subgraph cluster_1 {
        node [style=filled];
        "Item 3" "Item 4";
        label = "Container B";
        color=blue;
    }

    subgraph cluster_2 {
        node [style=filled];
        "Item 5" "Item 6";
        label = "Container C";
        color=blue;
    }

    // Renders fine
    "Item 1" -> "Item 2";
    "Item 2" -> "Item 3";

    // Both of these create new nodes
    cluster_1 -> cluster_2;
    "Container A" -> "Container C";
}

enter image description here

3
graphicsgraphvizdotdirected-graphsubgraph
I'm having the same problem, yet they have a natural example where subgraphs are acting like nodes, graphviz.org/content/fdpclust. - nlucaroni
@nlucaroni i wonder if this problem is solved. this example gives me wrong graph: edges connect centers of subgraph. don't you know how to make it to work like in the example? - k102
@k102, I do know. Check out that page again; it says you need to use fdp. The linked example, and the one above both work (the last line in the example here needs to use the subgraph names not the label and it might be nice to include line lengths for the graph); it's a little tight as is). - nlucaroni
@nlucaroni Using fdp v2.28.0 and copy/pasting the source from the example the lines connect to the center of the subgraph, not to the edges. If you open the .dot in OmniGraffle they are properly connected, while neato and dot both create superfluous nodes for the cluster. - Phrogz
Related Graphviz bug report: gitlab.com/graphviz/graphviz/-/issues/745 and others linked to from there. - Peter Nowee

3 Answers

226
votes

The DOT user manual gives the following example of a graph with clusters with edges between clusters:

IMPORTANT: The initial compound=true statement is required.

digraph G {
  compound=true;
  subgraph cluster0 {
    a -> b;
    a -> c;
    b -> d;
    c -> d;
  }
  subgraph cluster1 {
    e -> g;
    e -> f;
  }
  b -> f [lhead=cluster1];
  d -> e;
  c -> g [ltail=cluster0,lhead=cluster1];
  c -> e [ltail=cluster0];
  d -> h;
}

... and edges between nodes and clusters:

enter image description here

98
votes

For ease of reference the solution described in HighPerformanceMark's answer, applied directly to the original question, looks like this:

digraph G {

    graph [fontsize=10 fontname="Verdana" compound=true];
    node [shape=record fontsize=10 fontname="Verdana"];

    subgraph cluster_0 {
        node [style=filled];
        "Item 1" "Item 2";
        label = "Container A";
        color=blue;
    }

    subgraph cluster_1 {
        node [style=filled];
        "Item 3" "Item 4";
        label = "Container B";
        color=blue;
    }

    subgraph cluster_2 {
        node [style=filled];
        "Item 5" "Item 6";
        label = "Container C";
        color=blue;
    }

    // Edges between nodes render fine
    "Item 1" -> "Item 2";
    "Item 2" -> "Item 3";

    // Edges that directly connect one cluster to another
    "Item 1" -> "Item 3" [ltail=cluster_0 lhead=cluster_1];
    "Item 1" -> "Item 5" [ltail=cluster_0 lhead=cluster_2];
}

The compound=true in the graph declaration is vital. That produces output:

graph with connected clusters

Note that I changed the edges to reference nodes within the cluster, added the ltail and lhead attributes to each edge, specifying the cluster name, and added the graph-level attribute 'compound=true'.

Regarding the worry that one might want to connect a cluster with no nodes inside it, my solution has been to always add a node to every cluster, rendered with style=plaintext. Use this node to label the cluster (instead of the cluster's built-in "label" attribute, which should be set to the empty string (in Python, label='""'). This means I'm no longer adding edges that connect clusters directly, but it works in my particular situation.

12
votes

Make sure you are using fdp layout for the file. I don't think neato supports clusters.