simple rendering not working

[Anishek Agarwal <anishek AT gmail.com>]

Hello,

I am trying to render a simple graph with the code below

SingleGraph graph = new SingleGraph("test");
graph.addNode("1");
graph.addNode("2");
graph.addNode("3");
graph.addNode("4");

graph.addEdge("e1", "1", "2");
graph.addEdge("e2", "1", "3");
//    graph.addEdge("e3","2","4");
//    graph.addEdge("e4","3","4");

FileSinkImages sink =
    new FileSinkImages(FileSinkImages.OutputType.PNG, FileSinkImages.Resolutions.SVGA);
graph.write("/tmp/a.jpg");

 however all i am getting in the output JPG is a "dot"  as below. 

dependency is 

 <dependency>

    <groupId>org.graphstream</groupId>

    <artifactId>gs-core</artifactId>

    <version>1.3</version>

</dependency>

running on java 1.8 on mac OS Sierra

can someone help as to what am i doing wrong ?

thanks

anishek