Microsoft Reporting Services: how to separate bars in a series group

I have a column chart in a RDLC report that has a series grouping, so that each category shows two bars, like this:

I would like to add a small gap between each blue and red bar, so they do not touch. Is it possible? I've tested several properties but none seems to do this.

Tried literally everything, it seems it can't be done. Best you can do is add a dummy series group between the two "real" columns to simulate the extra space.