# Graphol: A Graphical Language for Ontology Modeling Equivalent to OWL 2
## 2 Related Work
Among other proposals, we below focus on GrOWL ([[http://growl.novasemantics.it/](http://growl.novasemantics.it/)]([http://growl.novasemantics.it/](http://growl.novasemantics.it/)), accessed on 17 January 2022) [52], and Graffoo ([[http://www.essepuntato.it/graffoo/](http://www.essepuntato.it/graffoo/)]([http://www.essepuntato.it/graffoo/](http://www.essepuntato.it/graffoo/)), accessed on 17 January 2022) [28] and VOWL ([[http://vowl.visualdataweb.org/](http://vowl.visualdataweb.org/)]([http://vowl.visualdataweb.org/](http://vowl.visualdataweb.org/)), accessed on 17 January 2022) [53], which, similar to ours, are specifically tailored to the representation and/or visualization of OWL ontologies. For a more comprehensive classification and comparison of available languages and tools for ontology editing and visualization, we refer the reader to [54]. GrOWL is a tool for visualizing and editing ontologies, based on the underlying DL semantics of OWL ontologies. GrOWL is able to map both TBox and ABox assertions to a graph-line representation through the use of color, shading, and different shape nodes to encode the properties of the language constructs. Although the core idea behind GrOWL is similar to Graphl's, the project seems to have been discontinued, and the available documentation does not provide an unambiguous indication of the syntax and semantics of the language. Differently to Graphl, OWLGrEd and Graffoo, GrOWL is also quite distant in nature from classical logical languages such as UML or ER, and this poses further difficulties in its understanding in industrial contexts. Graffoo is a graphical notation for OWL ontologies, developed using the standard library of the graph editor yEd ([[https://www.yworks.com/products/yed](https://www.yworks.com/products/yed)]([https://www.yworks.com/products/yed](https://www.yworks.com/products/yed)), accessed on 17 January 2022). The graphical elements featured in Graffoo are blocks (or nodes) and arcs. Blocks are used to model classes, datatypes, individuals, ontologies, and rules. Arcs are instead used to model assertions, annotation properties, attributes, and roles. Graffoo has been designed for fully capturing OWL 2, but to this aim, some elements of the language are not completely graphical. Indeed, those OWL 2 constructs that cannot be expressed by means of Graffoo's graphical elements are specified through OWL axioms in Manchester syntax. Therefore, the same arguments given for OWLGrEd about the need of embedding axioms given in a non-graphical way also apply to Graffoo. To the best of our knowledge, there is no editor tailored for Graffoo, but a palette for the yEd editor is available instead. VOWL (Visual Notation for OWL Ontologies) is a formalism that has been proposed quite recently. It defines a set of graphical primitives and a color scheme, and uses a force-directed graph visualization for the ontology. In VOWL, concepts (i.e., classes in OWL) are represented as circles, and data types are displayed in rectangles. A labeled arrow connecting two circles denotes a role (i.e., an objectProperty in OWL), whereas a labeled arrow connecting a circle to a datatype denotes an attribute (i.e., a dataProperty in OWL). The direction of the arrow establishes the typing of the domain and the range of the property. Cardinality constraints are specified on arrows in the style of UML class diagrams. The mentioned constructs are the basic elements of the representation. A complete list is available at [[http://vowl.visualdataweb.org/v2/](http://vowl.visualdataweb.org/v2/)]([http://vowl.visualdataweb.org/v2/](http://vowl.visualdataweb.org/v2/)) (accessed on 17 January 2022). VOWL is able to capture visually a good portion of OWL 2, although some constructs are not part of the actual VOWL visualization. The language is not defined through a formalized syntax, and in the documentation it is not clearly specified which fragment of OWL 2 is completely captured in a visual mode by VOWL.