DOT 语言
Wikipedia/DOT
Graphviz/The Dot Language
以下是定义 DOT 语言的抽象语法。关键字以粗体显示，非关键字以普通文本显示；关键字型的文字字符用单引号引用；括号（(、)）表示需要时分组；方括号（[、]）表示为可选项；垂直条（|）表示单选，即有且只能有一个。语法如下：
graph : [ strict ] (graph | digraph) [ ID ] '{' stmt_list '}'
stmt_list : [ stmt [ ';' ] stmt_list ]
stmt : node_stmt
| edge_stmt
| attr_stmt
| ID '=' ID
| subgraph
attr_stmt : (graph | node | edge) attr_list
attr_list : '[' [ a_list ] ']' [ attr_list ]
a_list : ID '=' ID [ (';' | ',') ] [ a_list ]
edge_stmt : (node_id | subgraph) edgeRHS [ attr_list ]
edgeRHS : edgeop (node_id | subgraph) [ edgeRHS ]
node_stmt : node_id [ attr_list ]
node_id : ID [ port ]
port : ':' ID [ ':' compass_pt ]
| ':' compass_pt
subgraph : [ subgraph [ ID ] ] '{' stmt_list '}'
compass_pt : (n | ne | e | se | s | sw | w | nw | c | _)
关键字node, edge, graph, digraph, subgraph, strict是大小写无关的。还要注意，允许的 compass_pt 的值不是关键字，所以这些字符串可以在其他地方用作普通标识符，相反，解析器实际上会接受任何标识符；
Layout
About / Graph Visualization / Layout
dot，hierarchical or layered drawings of directed graphs.
neato，”spring model” layouts.
fdp，stands for Force-Directed Placement.
sfdp，stands for Scalable Force-Directed Placement.
circo，circular layout.
twopi，radial layout.
nop，Pretty-print DOT graph file. Equivalent to nop1.
nop2，Pretty-print DOT graph file, assuming positions already known.
osage，draws clustered graphs.
patchwork，draws map of clustered graph using a squarified treemap layout.
Writing Layout Plugins，How to write a custom layout engine.