How do I set the Cluster font attributes in Graphviz?

I am able to开发者_开发问答 successfully change the edge font, and the node font, but my attempts to change the cluster labels have no affect. Can someone point me in the right direction?

I have tried labelfontname=, fontname= (on the edge and the node), but can't seem to find the magic formula for cluster labels.

fontname is working for cluster labels.

Here's an example with illustration - it's quite simple, just set it before setting the content of the label.

digraph g{
  node[fontname="Impact"];
  subgraph cluster0 {
    "Cluster node";
    fontname="Courier";
    label="Cluster label";
  }
  fontname="Arial";
  label="Graph Label";
}

The result: