Graphlet

Graphlet is a portable graph editor and toolkit for building graph editors, developed by Michael Himsolt at the University of Passau in the mid-to-late 1990s. It is the direct successor to Himsolt's earlier GraphEd system and is best known today as the birthplace of the GML file format (format:gml), which Himsolt designed as the editor's native serialization and later proposed at the GD'95 Graph Drawing Symposium as a common interchange format. Graphlet is distributed under the GPL/LGPL; its official site has long been marked "no longer available," though archived distribution files can still be found.

Architecturally, Graphlet is layered. At the bottom sits GTL, a C++ library for representing and manipulating graphs in the graph-theoretic sense (nodes, edges, directedness, traversals, algorithms). Above it, the editor exposes Graphscript, a scripting language built on Tcl/Tk and LEDA that drives customization, automation, and the implementation of graph algorithms without recompiling C++. Deferring all platform-dependent code to Tcl/Tk lets the same source run on UNIX and Microsoft Windows, and eases porting anywhere LEDA and Tcl/Tk exist. The bundled toolkit ships around seven layout algorithms, including random layout, several spring-embedder variants (GEM, Fruchterman-Reingold, Kamada-Kawai, and a constrained form), a DAG layer layout, and a tree layout.

In the graph-data and format-conversion landscape, Graphlet's lasting significance is GML rather than the editor itself. GML is a lightweight, 7-bit-ASCII, key/value list format for attributed directed and undirected graphs, and it was adopted well beyond Graphlet: LEDA, GraVis, VGJ, and later tools such as tool:networkx, tool:igraph, tool:gephi, and tool:graph-tool read or write it. GML also stands as a conceptual predecessor to the XML-based format:graphml, which the graph-drawing community developed afterward to address GML's limits around namespacing, validation, and extensibility.

The honest limitations are those of aging research software. Graphlet's Tcl/Tk-and-LEDA dependency chain, C++ build, and unmaintained status make it hard to install on modern systems, and it has effectively been superseded for interactive editing by tools like tool:yed, tool:gephi, and tool:cytoscape. Its enduring value is historical and format-centric: anyone working with .gml files is, in effect, using a standard that Graphlet defined.