This is really informative, and very well done to boot. Great work!<p>The colour scheme struck me as a little odd, though. It goes from darker (more migration) to lighter (less migration) but then abruptly to dark grey (no migration), making it harder to interpret at first glance. It would be nice if it were somewhat monotonic: bright colour = more migration, darker/duller colour = less migration, dark grey = no migration.