It isn't an illusion. That's what a truly random distribution looks like. There are techniques for producing a more even spread, nicely explained by Mike Bostoc

It isn't an illusion. That's what a truly random distribution looks like. There are techniques for producing a more even spread, nicely explained by Mike Bostock in the first part of http://bost.ocks.org/mike/algorithms/ http://bost.ocks.org/mike/algorithms/