If you like to save the image directly to a file instead of streaming it back to the browser then you just have to specify an absolute filename in the final call to Graph::Stroke(), i.e.
$graph-> Stroke( "/usr/home/peter/images/result2002.png");
Please note that the user running as Apache/PHP must have write access to the specified directory.
There are also two predefined filenames which have special meaning.
$handle = $graph->Stroke(__IMG_HANDLER );