Hi Christian, I've worked with customers to create some custom reports that show trends over past time periods (trailing 12 months, trailing 8 quarters, etc). This works best with tables and line charts. You can do this using the Time Period Range feature like so:
Select the interval you want to measure from the "Time" section of the project explorer window and drag it to the "columns" section of a report component.

Right click the time interval and select the "range" option.

Change the range to show prior periods.

Note: Apptio will always display the current period regardless of the time period range you select.
Note 2: Once you add a time period into the "columns" section of the component configurator, you will not be able to add in other fields to the "rows" or "values" section. I recommend adding the time period(s) you want to see as one of the last steps when building a chart or table on a report.
Hope that helps!