Hi Ken,
Perhaps I'm reading your chart wrong, but there seems to be a sharp decrease in both Yield and "Harvest Time over time." They are positively correlated, in the parlance of data geeks. I'm not entirely sure what "Harvest Time" means, but if it relates to flowering 12/12 light schedule it might be a clue. Did you start harvesting early?
You also mentioned that you began with phototropic plants and switched to autos. If that is the case your decreased yield isn't surprising, those pesty AutoFlowers are quick to harvest but stingy when it comes to harvest yield. Could your Photo->Auto switch be the contributing factor? Was your grow #6 or #8 the switch to AutoFlower seeds?
I'm probably missing something here, but that's what jumps out to me.
Regards,
-Grouchy