GraphML Progress Report?
Structural Layer Proposal
Ulrik Brandes1, Markus Eiglsperger2, Ivan Herman3, Michael Himsolt4, and
M. Scott Marshall3
1 Department of Computer & Information Science, University of Konstanz.
Ulrik.Brandes@uni-konstanz.de
2 Wilhelm Schickard Institute for Computer Science, University of Tubingen.
eiglsper@informatik.uni-tuebingen.de
3 Centrum voor Wiskunde en Informatica. fivan|scottg@cwi.nl
4 DaimlerChrysler Research. Michael.Himsolt@daimlerchrysler.com
Abstract. Following a workshop on graph data formats held with the
8th Symposium on Graph Drawing (GD 2000), a task group was formed
to propose a format for graphs and graph drawings that meets current
and projected requirements.
On behalf of this task group, we here present GraphML (Graph Markup
Language), an XML format for graph structures, as an initial step to-
wards this goal. Its main characteristic is a unique mechanism that allows
to dene extension modules for additional data, such as graph drawing
information or data specic to a particular application. These modules
can freely be combined or stripped without aecting the graph structure,
so that information can be added (or omitted) in a well-dened way.
1 Introduction
Graph drawing tools, like all other tools dealing with relational data, need to
store and exchange graphs and associated data. Despite several earlier attempts
to dene a standard, no agreed-upon format is widely accepted and, indeed,
many tools support only a limited number of custom formats which are typically
restricted in their expressibility and specic to an area of application.
Motivated by the goals of tool interoperability, access to benchmark data
sets, and data exchange over the Web, the Steering Committee of the Graph
Drawing Symposium started a new initiative with an informal workshop held
in conjunction with the 8th Symposium on Graph Drawing (GD 2000) [1]. As
a consequence, an informal task group was formed to propose a modern graph
exchange format suitable in particular for data transfer between graph drawing
tools and other applications.
On behalf of this group we propose GraphML (Graph Markup Language),
an XML format that takes a unique approach to represent graphs and graph
drawings by specifying
? The latest information on GraphML is maintained on the GraphML homepage [2].

1. core elements to describe graph structures together with
2. an extension mechanism that allows to independently build application-
specic graph data formats on top of them.
In particular, such extensions can be freely combined or ignored without af-
fecting the graph data itself. Thus, drawing information can be added to an
application-specic format, and graphs can be extracted from foreign applica-
tion data. These features seem to be essential requirements for today's and future
graph data formats, since graph models are ubiquitous and there will certainly
be no agreement on a single general format across all disciplines.
This report is organized as follows. In Sect. 2, we outline the guidelines used
in the design of GraphML. The core of the language is described in Sect. 3 and
in Sect. 4 we outline how to add non-structural data and thus bind GraphML
to specic applications. We conclude with future plans in Sect. 5.
2 Usage Scenarios and Design Goals
A modern graph exchange format cannot be dened in a monolithic way, since
graph drawing services are used as components in larger systems and Web-
based services are emerging. Graph data may need to be exchanged between
such services, or stages of a service, and between graph drawing services and
systems specic to areas of applications.
The typical usage scenarios that we envision for the format are centered
around systems designed for arbitrary applications dealing with graphs and other
data associated with them. Such systems will contain or call graph drawing ser-
vices that add or modify layout and graphics information. Moreover, such ser-
vices may compute only partial information or intermediate representations, for
instance because they instantiate only part of a staged layout approach such as
the topology-shape-metrics or Sugiyama frameworks. We hence aimed to satisfy
the following key goal.
The graph exchange format should be able to represent arbitrary graphs
with arbitrary additional data, including layout and graphics informa-
tion. The additional data should be stored in a format appropriate for
the specic application, but should not complicate or interfere with the
representation of data from other applications.
GraphML is designed with this and the following more pragmatic goals in mind:
{ Simplicity : The format should be easy to parse and interpret for both humans
and machines. As a general principle, there should be no ambiguities and thus
a single well-dened interpretation for each valid GraphML document.
{ Generality : There should be no limitation with respect to the graph model,
i.e. hypergraphs, hierarchical graphs, etc. should be expressible within the
same basic format.