I have solved my problem by recompiling Apache without PHP and GD works now without trouble. So I think, it's effectivly a library path problem with the original gd library used by GD and an another one added by PHP install process (with gd support). Why does GD works correctly sometimes still a great mystery for me ... What kind of debugging could I try ?
I have only checked Apache logs.
Thanks all for your help (and for Graph::Template that I will try soon !)