thanks for you quick answer
Yes of course I used the magic functions....
But in version 1.7.4, $objWriter->setIncludeCharts(true); makes php to fail (white page).
As I was using a quite old version of the library, I just upgraded to 1.8.
Now it works ... but the graph is altered. It has lost its title (which is a dynamic title refering to the first sheet).
The look and feel was also altered (see attached picture)
Note that Excel tells that the generated file is not correct and it has to recover it....(you know what I mean).
I thought this bug was fixed in the last versions of PHP Excel....
I also loose the background color of the graph if ever I defined one...