Cool! I've also been playing around with d3 lately so great to see some more examples.<p>One suggestion I'd have is to make the centre node something other than the word "English". Even as a native speaker I found it momentarily confusing (is this collocates for all English words?) so I could imagine it being more confusing for a non-native speaker.<p>Also it would be cool to put the collocates with the original word in the tool tip (i.e. "English professor" instead of just "professor") especially when the expanded coloured bubbles at the beginning are actually the collocates of the collocates.
In the description, I don't think "juxtapose" is the word you want to describe the relationship between the words. I am not a native speaker, but when I hear juxtapose, I think of things that are compared to each other, but with distance in mind, not closeness.