title: Ruby-Graphviz does not render png
body: <p>I just tried the ruby-graphviz gem (<a href="http://github.com/glejeune/Ruby-Graphviz" rel="nofollow noreferrer">http://github.com/glejeune/Ruby-Graphviz</a>). I followed the instructions (installed Graphviz, gem and dependencies) and tried the example from the Github page. Unfortunately I am not able to render any output image (png,dot).</p>
  
  <pre><code># Create a new graph
  g = GraphViz.new( :G, :type =&gt; :digraph )
  
  # Create two nodes
  hello = g.add_node( "Hello" )
  world = g.add_node( "World" )
  
  # Create an edge between the two nodes
  g.add_edge( hello, world )
  
  # Generate output image
  g.output( :png =&gt; "hello_world.png" )
  </code></pre>
  
  <p>When I run the skript from the console I get no error message but also no output as expected.</p>
  
  <p>What could be the problem? Folders have read/write access for everybody.</p>
  
  <p>Thanks in advance.  By the way, I´m working on a Mac (Leopard 10.6).</p>