My guess is you want to use Interactive Reporting because you are already using that to retrieve actuals information, or maybe you want to show this data in a nice Pivot or Report section.
You could code the 'extrapolate' logic using a Computed Item in your Results section, then use the Computed Item field in other sections (like a Pivot or Report). i.e. you could calculate in a computed item the % change in volume each year, and use that percentage to forecast next year values. Or, you could code similar logic using MS Excel. Even if you were using a Forecast/Planning tool you would need to build the requirements logic in to a calculation script or business rule.
Hope this helps,
I am using Hyperion IR vs 9
Computed Items will require intervention from me with manually governed growth rates (% increase/decrease). I'm actually looking for someting more sophisticated where there the slope of the growth(or decline) is calculated by a linear equation. Wasn't sure if using Hyperion IR is commonly used in this fashion to make future trend assumptions. I am certain I'm going to be asked by my constituents if it can.