OK, this is a very pretty picture. But, like I said before, it is hard to understand and to interpret.
It cannot be easy to visualize world air traffic when, on average, there are 93,000 daily flights originating from about 9,000 airports around the world. At any given time, there are between 8,000 and 13,000 planes in the air around the globe (link).
And to understand what the largest dots in these clusters are, take a look below:
For the rest of this list, check the Wikipedia.