Graphlet

Graphlet GML is the graph-file format of the Graphlet graph-editor system, developed by Michael Himsolt at the University of Passau in the mid-1990s as the successor to the earlier GraphEd toolkit. Graphlet itself is a portable, object-oriented editor and graph-drawing framework built on Tcl/Tk, LEDA and later the GTL (Graph Template Library), scripted through an extension language called Graphscript, and shipping layout algorithms such as spring embedders (GEM, Fruchterman-Reingold, Kamada-Kawai), DAG and tree layouts. Its native serialization is the Graph Modelling Language (GML), which Himsolt proposed at the 1995 Graph Drawing Symposium (GD'95) as a common, portable interchange format; GML subsequently spread far beyond Graphlet and is documented separately as format:gml.

Structurally, a Graphlet GML file is plain 7-bit ASCII built entirely from nested key-value pairs, with no reserved punctuation beyond whitespace and square brackets. A value is an integer, a floating-point number, a double-quoted string, or a bracketed list of further key-value pairs; this single recursive rule expresses the whole data model. The top level is a graph [ ... ] block whose list contains node [ id ... ] and edge [ source ... target ... ] entries, with a directed flag distinguishing digraphs. Nodes carry integer ids that edges reference, and shared keys like label and comment attach human-readable text. Visual state lives under graphics sublists holding coordinates, dimensions and rendering hints, so a Graphlet file round-trips both the abstract graph and its drawn layout.

The format's defining trait is open extensibility: any application may add its own keys anywhere in the tree, and a conformant reader simply ignores keys it does not understand. This let Graphlet, and later adopters, embed tool-specific attributes without breaking portability, but it also means GML defines a syntax and a small core vocabulary rather than a strict schema, so semantics of extended keys are convention-driven and vary between producers.

Within the format-conversion landscape, Graphlet GML occupies the same niche as other plain-text, attribute-carrying graph formats such as format:graphml, format:gexf-1.3, format:pajek, format:leda and format:dot, and is widely read and written by mainstream tools including tool:yed, tool:cytoscape, tool:gephi, tool:networkx and tool:igraph. Strengths are its human-readability, minimal grammar and layout-preserving round-tripping; limitations include the ASCII-only encoding, the lack of formal typing or namespacing for custom keys, and that the original Graphlet software is now effectively unmaintained, surviving mainly through the format it popularized.