Error using graphviz/phpopentracker

I wonder if anyone can help me! I am testing phpopentracker on a local machine using apache and php, and everything is going fine until i try and view clickpaths. When writing to the API engine: <?php include 'phpOpenTracker.php'; phpOpenTracker::get(array('api_call' => 'all_paths', 'result_format' => 'graphviz_object' )); ?> I am getting the error: Fatal error: Cannot instantiate non-existent class: phpopentracker_api_graphviz in c:\phpdev\www\phpOpenTracker\API.php on line 244 Somebody else in my organisation is getting the same problem, as I have realised on another mailing group. Does anyone know why this is? or what we are doing wrong? this problem is happening both locally, and on the server. I have installed the PEAR GraphViz script into the plugins folder, and installed the GraphViz software locally (does this have to be placed somewhere particular?). Would be very grateful if someone could help me with this problem. Thanks in advance James