[phpOpenTracker-general] Problem with Image/GraphViz.php

Hi.

I just installed phpOpenTracker and wanted to use it with jpgraph and
graphviz. But when I call the example of the documentation:

include "../../phpot/phpOpenTracker.php";

$graph = phpOpenTracker::get(

  array(

    'api_call'      => 'all_paths',

    'result_format' => 'graphviz_object'

  )

);
$graph->image('png');

I got the error 

"phpOpenTracker Error: Could not find PEAR
Image_GraphViz package, exiting."

I looked in the sources and it seems that this error occurs if the file
Image/GraphViz.php doesn't exists. And indeed, it doesn't exists. Where
can I get it. It does not have been delivered in the graphiz
installation package. 

Thanks for your help.

Best regards.

Roman