How do I add a WTD average to a nested bar chart?

I'm familiar with the min/max/average grid lines but they don't appear to work with a nested bar chart - or even with the stacked bar chart I changed it to.

I'm showing WTD inventory in lbs. across four orgs (series) for the last 7 days (date grain).

The business has asked for a total average inventory across all orgs by day.

Any thoughts on how to accomplish this or why I'm not seeing the min/max/average grid lines on my stacked bar chart?

Thanks!!
J.