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.
Introduction to GraphML 
A GraphML file consists of an XML file containing a
graph element, within which is an unordered sequence of
edge elements. Each
node element should have a distinct
id attribute, and each
edge element has
target attributes that identify the endpoints of an edge by having the same value as the
id attributes of those endpoints. Here is what a simple undirected graph with 2 nodes and one edge between them looks like:
<?xml version="1.0" encoding="UTF-8"?> <graphml xmlns="http://graphml.graphdrawing.org/xmlns" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd"> <graph id="G" edgedefault="undirected"> <node id="n0"/> <node id="n1"/> <edge id="e1" source="n0" target="n1"/> </graph> </graphml>
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.
Other Graph File Formats 
- GXL, graph exchange format based on XML -- GXL
- Trivial Graph Format, simple text based format
- GML is another widely used graph exchange format. -- GML
- DGML, Directed Graph Markup Language from Microsoft
- XGMML an XML-based graph markup language closely related to GML -- XGMML
- Dot Language, a format for describing graphs and their presentation, for the Graphviz set of tools.
See also 
- yEd, a widespread graph editor that uses GraphML as its native file format.
- Gephi, a graph visualization software that completely supports GraphML.
- http://graphml.graphdrawing.org/ homepage of GraphML
- http://graphml.graphdrawing.org/primer/graphml-primer.html An easy introduction to GraphML is the GraphML Primer.
- http://www.svgopen.org/2003/papers/ComparisonXML2SVGTransformationMechanisms/ Comparison between XML to SVG Transformation Mechanisms, showing conversions between GraphML and SVG
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