How to edit transparency on a report.

I would like to keep the report like this: 

However, when I select something in the slicer, it changes the graphic like this: 

 It works perfectly if theres nothing in the "size" field regardless if i have filtered something or not, i get the my desired result which is the first image. But once i put something in the "size" field and filter the slicer, i get the second image as a result.

What can I do? 

Thanks for any help.