Generates a description of graph in DOT file format. The
formatting can be altered using graph->dot-properties,
vertex->dot, and edge->dot as well as edge-formatter,
vertex-formatter, vertex-labeler, and edge-labeler. These can
be specified directly in the call to graph->dot or by creating
subclasses of basic-graph, basic-vertex and basic-edge.

The output can be a stream or pathname or one of the values nil or
t. If output is nil, then graph->dot returns a string containing
the DOT description. If it is t, then the DOT description is written
to standard-output.

Here is an example;

(let ((g (make-container 'graph-container :default-edge-type :directed)))
(loop for (a b) in '((a b) (b c) (b d) (d e) (e f) (d f)) do
(add-edge-between-vertexes g a b))
(graph->dot g nil))

“digraph G ”

For more information about DOT file format, search the web for 'DOTTY' and
'GRAPHVIZ'.