Bad Jasper Studio chart size in Excel ("fit to cell size" and not in "original size")

Question

I use Jaspersoft Studio 6.11.0 and when I export my report in excel (.xlsx) the charts size appear to be in "fit to cell size" and not "in original size".

I spend a lot of time to find hacks and parameters but I can't find a solution. I want to ensure that chart appear in it's original size in excel. The original size of the chart is 640x480.

I put a screen capture as you can easily see the problem

Thanks in advance.

screen capture here