[dbsmith83]

I think you could make it work for negative numbers. You would just shade the negative area differently to show it. For example, if you subtract 1000 sq ft from 100 sq ft, geometrically it means you are trying to remove more area than you have. So, to represent this, draw a rectangle of 100 sq ft. Conceptually "extend" this area by subtracting 1000 sq ft. The extra 900 sq ft that you try to subtract but can't "exists" as a negative representation. This could be represented by flipping the surplus 900 sq ft into a negative axis or shading it differently to denote that it's a deficit rather than an actual, positive area.