[phpOpenTracker-general] graphic is not created

Hi.

I installed phpOpenTracker and finally I got the first text-statistics.
Now I want to get graphical statistics. But when trying the example 

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

$graph = phpOpenTracker::get(

  array(

    'api_call'      => 'all_paths',

    'result_format' => 'graphviz_object'

  )

);

$graph->image('png');


there is image in the specified tmp-directory which is writable for
everyone. The jpg-config.inc.php has a configuration USE_CACHE. It was
set to false, I turned it on to true. This makes more sense.

Can anybody help me. I tried one of the jpgraph examples and there, a
file had been created in the specified directory. So the error must be
situated in the source of phpOpenTracker.

Thanks in advance for every hints.

Best regards.

Roman