Used Capacity Trending for VMAX Storage Group

Over the past week I've attempted to create a simple line graph for used capacity over time for a VMAX storage group with no success. I see a canned report for allocated capacity but nothing for used. I've copied out the storage group report and have manipulated it to ft my reporting needs so the used capacity sum is there, I just can't figure out how to graph it over a 3, 6 or 1 year period (which I can for allocated). I believe my issue is how I'm trying to sum the used capacity for the luns associated to the storage group. Allocated is easy as there is already a "capacity" property for the SG but used requires a bit more math that I can't seem to figure out. Any help would be greatly appreciated.