graphviz 的节点可以定义不同的外形，比如下面的定义，
digraph tt1{
a[shape=box];
c[shape=lpromoter];
d[shape=doubleoctagon];
a -> b;
c -> d;
d -> b;
}
会产生下面的图形：
具体 graphviz 支持的节点形状如下图：
来源：http://www.graphviz.org/doc/info/shapes.html
需要注意的是：
The geometries of polygon-based shapes are also affected by the node attributes regular, peripheries and orientation. If shape="polygon", the attributes sides, skew and distortion are also used. If unset, they default to 4, 0.0 and 0.0, respectively.
