[phpOpenTracker-general] trouble visualizing clickpaths

hello all,

i was able to solve my previous problem, thanks to sebastian's help.  
adding `ob_start();` before the first header() call in image.php kept 
the error message at bay.  unfortunately, i'm now having trouble getting 
graphviz to build the images to visualize click-paths:

i installed the simple_graph system w/o trouble, but when trying to get 
the clickpath visualization to work, i receive an error.  i setup a page 
as specified here: 
http://www.phpopentracker.de/docs/en/api-reference.api-calls.html 
(adding the requisite require(phpOpenTracker.php); ).
i try to load the page in my browser, and after sitting and thinking for 
a while, a blank page appears, and i receive the following error 
messages in the system logs:

   Sep 15 15:13:34 www httpd: PHP Warning:  fopen(): Unable to access 
/tmp/graph_KFDpBR.png in /usr/local/lib/php/Image/GraphViz.php on line 192
   Sep 15 15:13:34 www httpd: PHP Warning:    
fopen(/tmp/graph_KFDpBR.png): failed to open stream: No such file or 
directory in /usr/local/lib/php/Image/GraphViz.php on line 192


line 192 of GraphViz.php is the following:

       function fetch($format = 'svg') {
           if ($file = $this->saveParsedGraph()) {
               $outputfile = $file . '.' . $format;
               $command  = $this->graph['directed'] ? $this->dotCommand 
: $this->neatoCommand;
               $command .= " -T$format -o$outputfile $file";

               @`$command`;
               @unlink($file);

               $fp = fopen($outputfile, 'rb');

               if ($fp) {
                   $data = fread($fp, filesize($outputfile));
                   fclose($fp);
                   @unlink($outputfile);
               }
               return $data;
           }
           return FALSE;
       }

it seems that no file is being created, as the fopen is trying to read 
the file, and phpOpenTracker is writing temp files to /tmp without 
trouble.  is phpOpenTracker generating an image and placing it somewhere 
else?

not sure if this helps, but turning up the debugging level of 
phpOpenTracker gives the following when i run the clickpath php file:

   SELECT accesslog.accesslog_id AS accesslog_id,
                   accesslog.document_id  AS document_id,
                   accesslog.timestamp    AS timestamp,
                   documents.string       AS document,
                   documents.document_url AS document_url
              FROM pot_accesslog accesslog,
                   pot_visitors visitors,
                   pot_documents documents
             WHERE visitors.client_id    = '1'
               AND visitors.accesslog_id = accesslog.accesslog_id
               AND accesslog.document_id = documents.data_id
          ORDER BY accesslog.accesslog_id, accesslog.timestamp

thanks again, i really appreciate your help,

john B