如何使用graphstream

Question

这是我的代码，输出不是分层树的形式

public class Simple {

        public static void main(String[] args) {

            Graph graph = new SingleGraph("Test");

            Viewer viewer = graph.display(false);
            HierarchicalLayout hl = new HierarchicalLayout();
            viewer.enableAutoLayout(hl);
            graph.setStrict(false);
            graph.setAutoCreate(false); 
            graph.addNode("A" );        
            graph.addNode("B" );
            graph.addNode("C" );
            graph.addNode("D");
            graph.addNode("E");


        graph.getNode("A").addAttribute("ui.label", "A");
            graph.getNode("B").addAttribute("ui.label", "B");
            graph.addEdge("A-B", "A", "B",true);
            graph.addEdge("A-C", "A", "C",true);
            graph.addEdge("B-D", "B", "D",true);
            graph.addEdge("B-E", "B", "E",true);

即使我使用了分层布局，节点也是随机放置的。

我正在尝试绘制一棵树，如下图所示：