PEAR Graphviz：生成图像

Having installed the PEAR GraphViz package I'm trying to follow the second answer on this page to create a JPEG of a graph. As test code, I have the following:

<?php

  require_once 'Image/GraphViz.php';
  $graph = new Image_GraphViz();
  $graph->addNode('A');
  $img = $graph->fetch('jpeg', 'dot');
  print "IMG: " . $img;

I've also tried it using the $graph->image('jpeg', 'dot'); method. Both ways, it returns an empty string. When I change the format to something like SVG, it works, which makes me think that certain image generation libraries just aren't installed. But I have the regular (commandline) Graphviz tools installed and they generate images fine. What do I need to install to make this work so PHP can generate images itself?