Way to track switch port capacity?

Any ideas, or in-use methods, for how to track switch port capacity? 

By this I mean tracking how many ports are actually being used now and in historical time-frames?  Right now, this is a labor- & time-intensive manual process and is basically impossible to see any kind of trending. But it has an awful lot of value for a few reasons:

• being able to reclaim ports no longer in use to support additional wired connections is far less expensive than adding a new switch when demand for density n of new wired connections is generated -->new switch = purchase price + maintenance support + electricity + install labor
• being able to identify wired connections no longer in use can permit switch consolidation--> reducing the number of switches deployed lowers electricity bills, maintenance support bills and will have a big effect on reducing cost in the HW-refresh cycle
• switches are dense devices from a monitoring perspective; with fewer of them, collector deployments and resources can possibly be consolidated, reduced and/or simplified

I've cloned and then customized the Interfaces datasource and then adjusted the filtering to return ports that are down.  The default interfaces datasource filters returns (of course) ports that are up.  But this is where I have gotten stuck: So now some math (addition, division, multiplication) is required to arrive at a "capacity percentage" (ports up/total port count [ie ports down + ports up]) * 100 = capacity percentage  And then some way to keep a historical record in graphical or tabular format is needed.

I know this functionality exists out there in the market; thought I'd ask here to see if anyone has figured out how to do it on their own before exploring a feature request.

thanks.