Gregory,
I'm not an expert with this either, but I gave it a try and I think I got it, although my data sample doen't show much capacity change, but that's because I got only a very small amount of data. Anyway, I tried to indicate in the screenshot that I placed the vendor object from the physical storage capacity on the Y axis directly underneath the capacity object so to speak as a child object of the same.
Hope this is what you were looking for.
Regards,
Gerhard