The following code in our sandbox generates an empty graph:

[[!graph   src=""""
google [ href="" ]
sandbox [ href=[[SandBox]] ]
help [ href=[[ikiwiki/formatting]] ]
newpage [ href=[[NewPage]] ]

google -> sandbox -> help -> newpage -> help -> google;

It is the exact same thing that on the graph directive documentation, from the graphviz plugin. This is ikiwiki 3.20120203 on Debian wheezy and graphviz is installed (2.26.3-10). Note that the first demo actually works. See --anarcat

Looking at the example shows too many double quoted. fixed --Joey