Expression - comparing totals cost between 2 date ranges & calculating difference percentage

[#1]

Hi

I am looking to produce a straight table which shows by department a comparison between the total costs arising last month (-1) against the total cost arising  for the month  before (-2) and then show the difference percentage between the 2 months

I have the data set - 'costs' and 'creation date', 'creation month' and 'department' (see excel).  I would like to build an expression which totals all the costs with a creation date last month and compare this with the total cost with a creation date that falls in the prior month (-2)

How would I develop such an expression? and how do I ensure this continues to roll on as we progress though the year

Thanks

Rohan

[Gysbert_Wassenaar]

See attached example. If you simply add data for new months the expression will automatically show the last two months. If you select a month then it will show that month and the previous month.