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<TD ALIGN=RIGHT><I>compass_pt</I></TD>
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<TD ALIGN=LEFT>:</TD>
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<TD ALIGN=LEFT>(<B>n</B> | <B>ne</B> | <B>e</B> | <B>se</B> | <B>s</B> | <B>sw</B> | <B>w</B> | <B>nw</B>)</TD>
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<TD ALIGN=LEFT>(<B>n</B> | <B>ne</B> | <B>e</B> | <B>se</B> | <B>s</B> | <B>sw</B> | <B>w</B> | <B>nw</B> | <B>c</B> | <B>_</B>)</TD>
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The keywords <B>node</B>, <B>edge</B>, <B>graph</B>, <B>digraph</B>,
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<B>subgraph</B>, and <B>strict</B> are case-independent.
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Note also that the allowed compass point values are not keywords, so
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these strings can be used elsewhere as ordinary identifiers.
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these strings can be used elsewhere as ordinary identifiers and, conversely,
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the parser will actually accept any identifier.
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An <I>ID</I> is one of the following:
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<LI> Any string of alphabetic characters, underscores or
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digits, not beginning with a digit;
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<LI> a number [<tt>-</tt>]<sup>?</sup>(<tt>.</tt>[<tt>0</tt>-<tt>9</tt>]<sup>+</sup> | [<tt>0</tt>-<tt>9</tt>]<sup>+</sup>(<tt>.</tt>[<tt>0</tt>-<tt>9</tt>]<sup>*</sup>)<sup>?</sup> );
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<LI> Any string of alphabetic (<TT>[a-zA-Z\200-\377]</TT>) characters, underscores (<TT>'_'</TT>) or
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digits (<TT>[0-9]</TT>), not beginning with a digit;
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<LI> a numeral [<tt>-</tt>]<sup>?</sup>(<tt>.</tt>[<tt>0</tt>-<tt>9</tt>]<sup>+</sup> | [<tt>0</tt>-<tt>9</tt>]<sup>+</sup>(<tt>.</tt>[<tt>0</tt>-<tt>9</tt>]<sup>*</sup>)<sup>?</sup> );
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<LI> any double-quoted string ("...") possibly containing escaped
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quotes (\")<SUP>1</SUP>;
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<LI> an <A NAME=html>HTML string</A> (<...>).
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Note that in HTML strings, angle
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An ID is just a string; the lack of quote characters in the first two
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forms is just for simplicity. There is no semantic difference between
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<TT>abc_2</TT> and <TT>"abc_2"</TT>, or between <TT>2.34</TT> and
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<TT>"2.34"</TT>. Obviously, to use a keyword as an ID, it must be quoted.
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Note that, in HTML strings, angle
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brackets must occur in matched pairs, and unescaped newlines are allowed.
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In addition, the content must be legal XML, so that the special XML
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escape sequences for ", &, <, and > may be necessary
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that a named subgraph with no body immediately preceeds an anonymous
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subgraph, since the precedence rules cause this sequence to be parsed as
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a subgraph with a heading and a body.
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Also, any amount of whitespace may be inserted between terminals.
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As another aid for readability, dot allows single logical lines to
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span multiple physical lines using the standard C convention of a
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double-quoted strings can be concatenated using a '+' operator.
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As HTML strings can contain newline characters, they do not support the
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concatenation operator.
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<H2>Semantic Notes</H2>
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<H2>Subgraphs and Clusters</H2>
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Subgraphs play three roles in Graphviz. First, a subgraph can be used to
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represent graph structure, indicating that certain nodes and edges should
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be grouped together. This is the usual role for subgraphs
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and typically specifies semantic information about the graph components.
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In the second role, a subgraph can provide a context for setting attributes.
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For example, a subgraph could specify that blue
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is the default color for all nodes defined in it.
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graph drawing, a more interesting example is:
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rank = same; A; B; C;
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This (anonymous) subgraph specifies that the nodes A, B and C
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should all be placed on the same rank if drawn using dot.
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The third role for subgraphs directly involves how the graph
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will be laid out by certain layout engines. If the name of
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the subgraph begins with <TT>cluster</TT>, Graphviz notes the subgraph as
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a special <I>cluster</I> subgraph. If supported, the layout engine will
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do the layout so that the nodes belonging to the cluster are drawn together,
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with the entire drawing of the cluster contained within a bounding rectangle.
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Note that, for good and bad, cluster subgraphs are not part of the
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DOT language, but solely a syntactic convention adhered to by
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certain of the layout engines.
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<H2>Lexical and Semantic Notes</H2>
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If a default attribute is
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defined using a <B>node</B>, <B>edge</B>, or <B>graph</B> statement,
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or by an attribute assignment not attached to a node or edge, any object of the
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resetting the attribute as needed in the subgraphs, one can simple defer
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the attribute definition if the graph until the appropriate subgraphs
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have been defined.
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If an edge belongs to a cluster, its endpoints belong to that cluster.
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Thus, where you put an edge can effect a layout, as clusters are sometimes
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laid out recursively.
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There are certain restrictions on subgraphs and clusters. First, at
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present, the names of a graph and it subgraphs share the same namespace.
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Thus, each subgraph must have a unique name. Second, although nodes
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can belong to any number of subgraphs, it is assumed clusters form
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a strict hierarchy when viewed as subsets of nodes and edges.
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<H2>Character encodings</H2>
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The DOT language assumes at least the ascii character set.
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Quoted strings, both ordinary and HTML-like, may contain non-ascii characters.
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into a string, one can use the ascii sequence <TT>&beta;</TT>.
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In general, one should only use entities that are allowed in the output
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character set, and for which there is a glyph in the font.
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<LI> In quoted strings in DOT, the only escaped character is double-quote
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("). That is, in quoted strings, the dyad \" is converted to "; all other
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characters are left unchanged. In particular, \\ remains \\. Layout
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engines may apply additional escape sequences.
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doc/info/lang.html
