This is not easy to do, so the solutions depend on the nature of what you are trying to summarize. The issue is that you cannot natively do arithmetic calculations on columns in a summary report.
If, for example, you just wanted grand totals for the whole Company year over year, then you would need ot load up a table of say the next 20 years, set the Key field to be the year and then using summary fields get to totals you need up to that record. Then you can do all the math you like.
If these were totals by say Customer, then you would need to have summary fields such as say YTD sales this year, and YTD sales last year (say to the most recently completed month) and then do the math on the customer record.
