I needed to see the dependencies between the ant tasks in our project.
I first came across this older page with different tools for creating a graph of the dependencies, but unfortunately, the tools mentioned here do not seem to be developed any more.
But then I found this great groovy script embedded as an ant target.
I downloaded groovy, and separated the taskdef from the groovy script, in order to place the groovy libraries where I wanted.

   <taskdef name="groovy" classname="org.codehaus.groovy.ant.Groovy">
     <classpath>
	<fileset dir="groovy-1.8.6/lib" includes="*.jar"/>
      </classpath>
    </taskdef>
    <groovy>
      ....
    </groovy>

GraphViz was installed in no time (from ubuntu APT), and the resulting graph is quite good!