GraphML is an XML-based file format for graphs.
The GraphML file format results from the joint effort of the graph drawing community to define a common format for exchanging graph structure data.
It uses an XML-based syntax and supports the entire range of possible graph structure constellations including directed, undirected, mixed graphs, hypergraphs, and application-specific attributes.
[1] A GraphML file consists of an XML file containing a graph element, within which is an unordered sequence of node and edge elements.
Here is what a simple undirected graph with two nodes and one edge between them looks like: Additional features of the GraphML language allow its users to specify whether edges are directed or undirected, and to associate additional data with vertices or edges.