Very neat visualization! Note that these charts are of static allocations, not dynamic usage. Dynamically, we use very little of the spectrum that we've statistically allocated, even in a busy urban environment: http://www.sharedspectrum.com/wp-content/uploads/2010_0923-G... (see Figure 9 for the punchline). Look at all that open real estate, and think about how much we've managed to do with a measly 100 MHz in the ISM band.
I'm not so sure this is actually "available" spectrum, as it is unused-by-carrier spectrum.
For example: the large dark spot in New Mexico is probably that way for a reason: The VLA is smack in the middle.