There is only one file type that uses the .XGML extension:
Graph Modeling Language XML File
File created in the XML version of the Graph Modeling Language (GML) format (the plain ASCII version uses the .GML extension); used to describe graph structures such as hierarchies, networks, and knowledge relationships.
XGML files are used as the XML-parseable equivalent of the ASCII text .GML format.
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Updated: April 1, 2014