使用shape="circle" 应该可以工作,例如:
digraph G {
0 [color="aqua", label="A", shape="circle", style="filled"];
1 [color="bisque", label="B", shape="circle", style="filled"];
2 [color="blue", label="C", shape="circle", style="filled"];
3 [color="blueviolet", label="E", shape="circle", style="filled"];
4 [color="brown", label="D", shape="circle", style="filled"];
5 [color="burlywood", label="F", shape="circle", style="filled"];
6 [color="cadetblue", label="G", shape="circle", style="filled"];
0->0 ;
0->1;
1->4;
2->2;
2->3;
3->4;
4->4;
4->5;
4->6;
5->0;
6->2;
}