Graphviz: An Open Source Graph Visualization Software

Graphviz (short for Graph Visualization Software) is a package of open-source tools for creating graphs. It takes text input in DOT format, generates images.

DOT Format

DOT is a graph description language. DOT files are usually with .gv filename extension.

See the official document for the complete DOT format specification. Here I use an example directed graph to show the basic concepts of the format:

// File `test.gv`:
digraph TestGraph {
    // Graph (or global) attributes
    bgcolor = "lightgrey";
    /**
     * Node name can be characters or digits
     */
    a [fontcolor = "red";color = "green";];
    c [shape = "triangle";];
    a -> b [arrowhead = diamond; color = blue;];
    b -> b;
    a -> 12;
    a -> 12;
    a -> c -> a;
}

The DOT file produces the following image with commands:

# Install Graphviz package if you haven't
sudo apt install graphviz
# Generate image from DOT file
dot -Tpng test.gv > test.png

Here comes some rules of the DOT format that are reflected in the example:

• A directed graph begins with key word digraph. An undirected graph begins with graph (not for this example).
• An edge connects 2 nodes (like a->b). Multiple connections can also be concatenated, like a->c->a.
• Node name can contain characters, digits and symbols. Digits-only name is also acceptable.
• In a directed graph, the nodes are connected with ->. While in the undirected graph, the edges should be --.
• Attributes are represented with key-value pairs.
• Graph (or global) level attributes are written in the body of the graph. Node attributes are written after a stand-alone node definition. Edge attributes are written after the connection statement.
• Comments can be either // or /* ... */

Layout Engine

The DOT format only describe the relation of nodes, edges and their attributes, but doesn’t determine how the nodes and edges are placed (the layout) in a picture.

When you generate the image from a DOT file, there are different engines to choose:

• dot : Hierarchical or layered drawings of directed graphs
• neato : “Spring model” layouts
• fdp : Force-Directed Placement
• sfdp : Scalable Force-Directed Placement
• circo : Circular layout
• twopi : Radial layout
• nop : Pretty-print DOT graph file. Equivalent to nop1
• nop2 : Pretty-print DOT graph file, assuming positions already known
• osage : Draws clustered graphs
• patchwork : Draws map of clustered graph using a squarified treemap layout

After installing the Graphviz software package, each engine is available as an installed binary. The filename is same with the name of the engine.

The following diagram shows the work of different engines on the example DOT file.

Tools

In my VS Code editor, I installed an extension called Graphviz Interactive Preview, which is quite convenient.

Here is a screenshot of the tool when I was creating the example DOT file. The tool provides some popup menus for the ease of editing. And the preview updates in real-time.