Modifying Graphviz Edge Labels with GVPR

I recently made a Graphviz graph with long edge label text and I was unhappy with the layout results. The text was often placed too far from an edge to make it clear which edge it was attached to. The decorate attribute helped, but was ugly.

To make the diagram more clear, I moved the label text into new nodes in the middle of the edge. I automated this with GVPR, part of the Graphviz tools.

GVPR is a neat language heavily influenced by Awk designed to modify graphs notated in Graphviz. It walks through all the edges and nodes of a graph for you, so your code can focus on the operations you want to perform, not how to traverse the graph.

This script was a little tricky to figure out. Modifying the graph in place would cause infinite loops, because splitting an edge in two made two more edges for the E pattern matcher to process. So I had to learn how to copy the input graph to the output graph ($O) as it iterated over the input.

For example, here's the bridges of Koenigsberg:

And the finite state machine example from the Graphviz gallery:

Here's the source code:

label_box.gvpr:

/*
  Copyright (C) 2025 Remington Furman

  Permission is hereby granted, free of charge, to any person
  obtaining a copy of this software and associated documentation files
  (the "Software"), to deal in the Software without restriction,
  including without limitation the rights to use, copy, modify, merge,
  publish, distribute, sublicense, and/or sell copies of the Software,
  and to permit persons to whom the Software is furnished to do so,
  subject to the following conditions:

  The above copyright notice and this permission notice shall be
  included in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
  BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.

  Replace edge labels with rectangular nodes that contain the label
  text.

  Inspired by:
  https://forum.graphviz.org/t/how-to-tell-dot-to-put-the-edge-label-in-a-box/1578/8
  and:
  https://www.exchangetuts.com/how-can-i-reverse-the-direction-of-every-edge-in-a-graphviz-dot-language-graph-1640404624170374
*/

BEG_G {
  string nodeName, prefix="__LabelNode_";
  int I=0;
  node_t labelNode;
  edge_t begEdge;
  edge_t endEdge;
  $O = graph($.name + "_box", "D");
  copyA($G, $O);
}

N {
  clone($O, $);
}

E {
  if (hasAttr($,"label")){
    I++;
    printf("edge %d, %s\n", I, $.label);
    nodeName = prefix+(string)I;
    labelNode = node($O,nodeName);
    labelNode.label = $.label;
    labelNode.shape="rect";
    begEdge = edge_sg($O, clone($O, $.tail), labelNode, "");
    endEdge = edge_sg($O, labelNode, clone($O, $.head), "");
    switch ($.dir) {
      default:
      case "forward": {
        begEdge.dir = "none";
        endEdge.dir = "forward";
        break;
      }
      case "back": {
        begEdge.dir = "back";
        endEdge.dir = "none";
        break;
      }
      case "both": {
        begEdge.dir = "back";
        endEdge.dir = "forward";
        break;
      }
      case "none": {
        begEdge.dir = "none";
        endEdge.dir = "none";
        break;
      }
    }
  } else {
    copy($O, $);
  }
}

END {
  printf("done.\n")
}

And a Makefile for using it:

DOTS=$(wildcard *.dot)

SVGS=$(patsubst %.dot,%.svg,$(DOTS))
LABELED_SVGS=$(patsubst %.dot,%_label.svg,$(DOTS))
PNGS=$(patsubst %.dot,%.png,$(DOTS))
LABELED_PNGS=$(patsubst %.dot,%_label.png,$(DOTS))

all: $(SVGS) $(LABELED_SVGS) $(PNGS) $(LABELED_PNGS)

%.svg: %.dot
        dot -Tsvg $< > $@

%.png: %.dot
        dot -Tpng $< > $@

%_label.dot: %.dot label_box.gvpr
        gvpr -o $@ -f label_box.gvpr $<