Hi,
yes I've seen that before, the expansion board will not help you with such a project.
You need to convert the 20 levels of each vertical axis from the spectrum analyzer to 20 voltage levels so that when all 20 levels are on (20 high bits) you will have your max voltage (example 12V).
The spectrum analyzer wasn't build with this in mind so you will have allot of work to do. Unfortunately I don't have the time to help you with your project but if you are a student you should be able to get help from your professors.
Good luck.

