rdf validator does not produce graphical output

Hi,

I try to get any graphical output, but I get only the message "The graph image 
file is empty." even if I try the default example.

any hints on how to get a graph?

I tried also an example from de.wikipedia:

<?xml version="1.0" encoding="UTF-8" ?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns:dc="http://purl.org/dc/elements/1.1/">
  <rdf:Description 
rdf:about="http://de.wikipedia.org/wiki/Resource_Description_Framework">
    <dc:title>Resource Description Framework</dc:title>
    <dc:publisher>Wikipedia - Die freie Enzyklopädie</dc:publisher>
  </rdf:Description>
</rdf:RDF>

best wishes,
Stefan Farrenkopf