How many ICAO addresses for aircraft are there?

[traceCraft]

Well, we know the total number of possible addresses to be used by aircraft is 2^24. This is an enormous space of 16,777,214 hex codes. However, many are not assigned, including whole blocks which are reserved. Some countries are having un-allocated addresses close to 99% of their quota. So, I wondered how many are active. I have tried to count in various databases and also in my own radiolocation logs. The result is graphically represented here (give it some time to load). Any ideas how to progress further?