Table 4. Comparison between knowledge graph visualization options.

	Open source	RDF support	SPARQL Support
Popoto.js	Open source licensed under GNU General Public License v3.0	NA	JavaScript naturally fits for querying a SPARQL endpoint which provides a REST service returning the result in the JSON format
D3.js	open source licensed under BSD license.	NA`	JavaScript naturally fits for querying a SPARQL endpoint which provides a REST service returning the result in the JSON format
Neo4j Bloom	open source licensed under GPLv3.	Yes, via the neosemantics (n10s) plugin.	No, Neo4j has Cypher that covers the need for a structured graph query language.
Grafo	No. offers a Free/Student tier with basic functionality.	supports importing OWL (RDF/XML) and Turtle file formats.	No
WebVOwl	Open source released under the MIT license	visualizations are automatically generated from JSON files into which the ontologies need to be converted	Yes