Diagramming an RDF/XML OWL ontology

Over the course of time (many times, in fact), I have been asked to "graph" my ontologies to help visualize the concepts. Anyone who has worked with Protege (or the Neon Toolkit or other tools) knows that none of the tools give you all the images that you really need to document your work. I have often resorted to hand-drawing the ontologies using UML diagrams. This is both painful and a huge time sink.

Recently, I was reading some emails on the Linked Data community distribution list about how they generate the LOD cloud diagram. Omnigraffle is used in the "official" workflow to create this diagram, but that tool costs money to buy. One of the email replies discussed a different approach.

A gentleman from Freenet.de needed to draw a similar diagram for the data cloud for the Open Linguistics Working Group. His team could not use the same code and processing flow as the LOD cloud folks, since they didn't have many Mac users. So, they developed an alternative based on GraphML. To create the basic graph, they developed a Python script. And, ...

Using yed's "organic" layout, a reasonable representation can be achieved which is then manually brought in shape with yed (positioning) and XML (font adjustment). In yed, we augment it with a legend and text and then export it into the graphic format of choice.

Given my propensity to "reuse" good ideas, I decided to investigate GraphML and yEd. And, since GraphML is XML, ontologies can be defined in RDF/XML, and XSLT can be used to transform XML definitions, I used XSLT to generate various GraphML outputs of an ontology file. Once the GraphML outputs were in place, I used yEd to do the layout, as the Freenet.de team did. (It is important to note that the basic yEd tool is free. And, layout is the most difficult piece of doing a graphic.)

So, what did I find? You can be the judge. The XSLTs are found on GitHub (check out http://purl.org/NinePts/graphing). There are four files in the graphing directory:

• AnnotationProperties.xsl - A transform of any annotation property definitions in an RDF/XML file, drawing them as rectangles connected to a central entity named "Annotation Properties".
• ClassHierarchies.xsl - A transform of any class definitions in an RDF/XML file, drawing them in a class-superclass hierarchy.
• ClassProperties.xsl - A transform of any data type and object property definitions in an RDF/XML file, drawing them as rectangles with their types (functional, transitive, etc.) and domains and ranges.
• PropertyHierarchies.xsl - A transform of any data type and object property definitions in an RDF/XML file, drawing their property-super property relationships.

I executed the transforms using xsltproc. An example invocation is:

xsltproc -o result.graphml ../graphing/ClassProperties.xsl metadata-properties.owl

I then took the result.graphml and opened it in the yEd Graph Editor. (If you do the same, you will find that all the classes, or properties lay on top of each other. I made no attempt to do any kind of layout since I planned to use yEd for that purpose.) For the class properties graph (from the above invocation), I used the Layout->Radial formatting, with the default settings. Here is the result:

I was impressed with how easy this was!

The really great thing is that if you don't like a layout, you can choose another format and even tweak the results. I did some tweaking for the "Property Hierarchies" diagram. In this case, I ran the PropertyHierarchies.xsl against the metadata-properties.owl file and used the Hierarchical Layout on the resulting GraphML file. Then, I selected all the data properties and moved them underneath the object properties. Here is the result:

Admittedly, the diagrams can get quite complex for a large ontology. But, you can easily change/combine/separate the XSLT transforms to include more or less content.

With about a day and half's worth of work (and using standards and free tooling), I think that I saved myself many frustrating and boring hours of diagramming. Let me know if you find this useful, or you have other suggestions for diagramming ontologies.

Andrea