I'm not sure that you can capture inventory trends over time. I think that what you would need to do is create a report of the data that you want (such as server count) and export it on a regular basis, like once a week. Then you could create reports of server count totals or whatever over time in Excel.
I believe inventory trends are possible with BSARA, the new Cognos reporting tool that is coming out in the next month for customers on at least version 7.4.3. (But make sure you know for sure before you tell the customer.)