An Introduction to GraphViz and dot
Pages: 1, 2

Basic Concepts of dot

A generic dot graph is composed of nodes and edges. Our hello.dot example contains a single node and no edges. Edges enter in the game when there are relationships between nodes, for instance hierarchical relationships as in this example, which produced Figure 3:

digraph simple_hierarchy {

B [label="The boss"]      // node B
E [label="The employee"]  // node E

B->E [label="commands", fontcolor=darkgreen] // edge B->E

}

Figure 3. A hierarchical relationship

dot is especially good at drawing directed graphs, where there is a natural direction. (GraphViz also includes the similar neato tool to produce undirected graphs). In this example the direction is from the boss, who commands, to the employee, who obeys. Of course dot gives you the freedom to revert social hierarchies, as seen in Figure 4:

digraph revolution {

B [label="The boss"]      // node B
E [label="The employee"]  // node E

B->E [label="commands", dir=back, fontcolor=red]  
// revert arrow direction 

}

Figure 4. An inverted hierarchy

Sometimes, you want to put things of the same importance on the same level. Use the rank option, as in the following example, which describes a hierarchy with a boss, two employees, John and Jack, of the same rank, and a lower ranked employee Al who works for John. See Figure 5 for the results.

digraph hierarchy {

nodesep=1.0 // increases the separation between nodes

node [color=Red,fontname=Courier]
edge [color=Blue, style=dashed] //setup options

Boss->{ John Jack } // the boss has two employees

{rank=same; John Jack} //they have the same rank

John -> Al // John has a subordinate 

John->Jack [dir=both] // but is still on the same level as Jack
}

Figure 5. A multi-level organizational chart

This example shows a nifty feature of dot: if you forget to give explicit labels, it will use the name of the nodes as default labels. You can also set the default colors and style for nodes and edges respectively. It is even possible to control the separation between (all) nodes by tuning the nodesep option. I'll leave it as an exercise for the reader to see what happens without the rank option (hint: you get a very ugly graph).

dot is quite sophisticated, with dozen of options which you can find in the excellent documentation. In particular, the man page (man dot) is especially useful and well done. The documentation also explains how to draw graphs containing subgraphs. However, those advanced features are outside the scope of this brief article.

We'll discuss another feature instead: the ability to generate output in different formats. Depending on your requirements, different formats can be more or less suitable. For the purpose of generating printed documentation, the PostScript format is quite handy. On the other hand, if you're producing documentation to convert to HTML format and put on a Web page, PNG format can be handy. It is quite trivial to select an output format with the -T output format type flag:

$ dot hello.dot -Tpng -o hello.png

There are many others available formats, including all the common ones such as GIF, JPG, WBMP, FIG and more exotic ones.

Generating dot Code

dot is not a real programming language, but it is pretty easy to interface dot with a real programming language. Bindings exist for many programming languages—including Java, Perl, and Python. A more lightweight alternative is just to generate the dot code from your preferred language. Doing so will allow you to automate the entire graph generation.

Here is a simple Python example using this technique. This example script shows how to draw Python class hierarchies with the least effort; it may help you in documenting your code.

# dot.py 

"Require Python 2.3 (or 2.2. with from __future__ import generators)"

def dotcode(cls):
    setup='node [color=Green,fontcolor=Blue,fontname=Courier]\n'
    name='hierarchy_of_%s' % cls.__name__
    code='\n'.join(codegenerator(cls))
    return "digraph %s{\n\n%s\n%s\n}" % (name, setup, code)

def codegenerator(cls):
    "Returns a line of dot code at each iteration."
    # works for new style classes; see my Cookbook
    # recipe for a more general solution
    for c in cls.__mro__:
        bases=c.__bases__
        if bases: # generate edges parent -> child
            yield ''.join([' %s -> %s\n' % ( b.__name__,c.__name__)
                           for b in bases])
        if len(bases) > 1: # put all parents on the same level
            yield " {rank=same; %s}\n" % ''.join(
                ['%s ' % b.__name__ for b in bases])

if __name__=="__main__": 
    # returns the dot code generating a simple diamond hierarchy
    class A(object): pass
    class B(A): pass
    class C(A): pass
    class D(B,C): pass
    print dotcode(D)

The function dotcode takes a class and returns the dot source code needed to plot the genealogical tree of that class. codegenerator generates the code, traversing the list of the ancestors of the class (in the Method Resolution Order of the class) and determining the edges and the nodes of the hierarchy. codegenerator is a generator which returns an iterator yielding a line of dot code at each iteration. Generators are a cool recent addition to Python; they come particularly handy for the purpose of generating text or source code.

The output of the script is the following self-explanatory dot code:

digraph hierarchy_of_D {

node [color=Green,fontcolor=Blue,font=Courier]

 B -> D
 C -> D

 {rank=same; B C }

 A -> B

 A -> C

 object -> A

}

Now the simple one-liner:

$ python dot.py | dot -Tpng -o x.png

generates Figure 6.

Figure 6. A Python class diagram

References

You may download dot and the others tool coming with GraphViz at the official GraphViz homepage. You will also find plenty of documentation and links to the mailing list.

Perl bindings (thanks to Leon Brocard) and Python bindings (thanks to Manos Renieris) are available. Also, Ero Carrera has written a professional-looking Python interface to dot.

The script dot.py I presented in this article is rather minimalistic. This is on purpose. My Python Cookbook recipe, Drawing inheritance diagrams with Dot, presents a much more sophisticated version with additional examples.

Michele Simionato is employed by Partecs, an open source company headquartered in Rome. He is actively developing web applications in the Zope/Plone framework.

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