Sounds like you may need to utilize calculations to do the math for this. Here is a good location to help get you rolling with Calculations.
I think you need MI (Management Information) to do what you want. Dashboards are filled with lists, simple graphs of sums, or sums of lists (i.e. Qty Incidents created today). Their not good at looking at percents or differences between values. I believe MI can do that and more. Your other choice is to put a graph in a web site and then publish the web site as a gadget in a dashbaord. You can also use Crystal Reports Server to create reports that can be published as web pages although that is not fun to do. Might want to look at MI because I think for the info yoru looking at, it's difficult to do.
The dashboards don't seem to be a place to look at sophisticated management reports, their more a place to show what still needs to be done and how much did we do. For management reports we get Crystal Reports where you can do the type of things you are probably looking for. CR may be a pain in the butt to learn, to use, etc etc, but it can do a lot and with enough effort amazing things can be done. I have a crystal reports server that makes interactive management reports to create fastastic detail and flexible reports. But oh is it horrible to work with and learn. Some of the worst documentation and support.