Crosstab: Variance on same item

Good day. I am in need of assistance on an issue I'm having with a Discoverer crosstab calculation.

My items are:

Left axis: Expense Reports.Department

Top axis: Expense Reports.Account

                Invoice Distributions.Accounting Date - Year

Data Point: Expense Reports.Line Amount SUM

------------------------------

Therefore, my report comes over as (items in parantheses are just for labelling purposes):

                                  001800      (Account#)

                                  2010           2011       (Years)

3200 (Dept#)            1,000.00       1,500.00

3211 (Dept#)               750.00       1,000.00

3212 (Dept#)               500.00          500.00

-------------------------------

I'm trying to get:

                                  001800      (Account#)

                                  2010           2011       (Years)    Difference

3200 (Dept#)            1,000.00       1,500.00                  500.00

3211 (Dept#)               750.00          500.00                <250.00>

3212 (Dept#)               500.00          500.00                      0.00

----------------------------------

I'm trying to create a variance column to provide the difference from the year 2010 to the year 2011. However, since these years are coming in from the same item (Invoice Distributions.Accounting Date - Year), I've read that I need to create separate items for each using a calculation. Does anyone know how to accompish this? I've tried using LAG, DECODE, etc and either those functions do not work or my calculation is missing something.