Take the frequency allocation chart at http://www.ntia.doc.gov/osmhome/allochrt.pdf
and illuminate each block when a receiver detects (narrowband) activity in each block.

That would be cool. Its just a quantization of a (broadband) spectrum analyzer's output. It needn't update in fast real time, just as it scans 10,000-10,000,000,000 hz in whatever steps.