jpgraph problem

Hi,

I'm trying to get phpOpenTracker to work on my server. I've got it all 
installed and working properly with the exception of the graph plotting. 
I have followed the installation instructions of jpgraph and those 
provided with phpOpenTracker exactly (or so I believe) but when I try to 
call one of the plot functions, I get this error:

Fatal error: Cannot instantiate non-existent class: piegraph in 
/usr/local/lib/php/phpOpenTracker/API/plugins/plot_top.php on line 119

This is using the code

<?php
phpOpenTracker::plot(
   array(
     'api_call' => 'top',
     'what'     => 'document',
     'limit'    => 10,
     'range'    => 'current_month'
   )
);
?>

Are you able to help?
-- 
Regards,

David Mytton

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