System Discount not reflecting in Forecast

I want to create a forecast report which not only gives me the total profit of a particular group of sales and/or proposals, but also breaks down by line items. I have tried, for instance, modifying the "Forecast by Item Details" report, and I can get everything I want EXCEPT for a reflection of the system discount. Obviously this has a significant effect on the final profit number, so its absence is painful. I seem to be able to get correct high-level transaction summaries for proposals, but it appears that only Sales Orders and Invoices can be reported to the level of detail I want.