I've written quite a bit about flow mapping on these blog pages. It's only been a sideline of mine in terms of research but has consumed a lot of my time over the past year or so (well, more like three or four). So, one final image, embedded in Google Maps. This one shows gross flows of migrants between Glasgow wards from 2000 to 2001 (gross flows = two-way; that is, the total flow in both directions between two wards). Click on individual lines for more information. If you want to see the map full size in Google Maps, click the link below this smaller version and it will open.
View Larger Map