I understand how to normalize a relational database. You will find methods for dealing with a fifth normal form. I realize why you might want to back away to 4th normal or else.

What's the equivalent way of an ontology which describes a graph?

I'm not conscious of any mechanism for ontologies that's directly similar to database normalization. The nearest match I'm able to think about are ontology design designs. However, they're a smaller amount strict. You are able to roughly do a comparison to software design designs. You should check

http://ontologydesignpatterns.org/wiki/Main_Page

or take a look at some papers, e.g., concerning the M3O (http://dl.acm.org/citation.cfm?id=1772775), Event Model F or by Aldo Gangemi, among many more. Ontology design designs also provide you with certain qualities, however they mainly rely on the designs you utilize, and which of them work is dependent around the modeling taks you attempt to attain.

Both design designs and database normalization attempt to achieve certain qualities. I suppose the main difference is, that design designs are less strict. The accomplished qualities are frequently with respect to the designs you utilize, the domain, the reason etc. So, they aren't really as generic because the normal forms.