type: escString, default:
Allows the graph author to provide an identifier for graph objects which is to be included in the output.
\G substitutions are applied.
If provided, it is the responsibility of the provider to keep
id values unique for its intended downstream use.
Note, in particular, that
\E does not provide a unique id for multi-edges.
id attribute is provided, then a unique internal id is used. However,
this value is unpredictable by the graph writer.
If the graph provides an
id attribute, this will be used as a prefix for
internally generated attributes. By making internally-used attributes
distinct, the user can include multiple image maps in the same document.