I think you first need to think about how you would like to define 'close together' and 'far apart' in practical terms, and when you've worked that out, write some code.

I'm happy to be corrected if I'm missing something...

That's why I had $avg - I don't really like taking the average distance to do this (I'd much prefer to have a score system where I get everything and then filter out after) but as I can't even figure this out, I figure this is a good starting point. I could work with it if I got this working at least.