Thanks for taking the time to explain the visualization (I didn't build this, but helped with some of the bugfixes). We're super interested in looking at this kind of data, and it's not quite the easiest to explain. There are a lot of small cases that would disappear if we were only looking at the net inbound/outbound, but adding it as a third mode is a good idea.

For reference, we also have a companion blog post to this visualization here: http://insights.truliablog.com/2012/02/where-are-house-hunte...

And a longer post on an index based around that net inbound/outbound searching, what we're calling the Metro Movers Index: http://insights.truliablog.com/2011/11/trulia-metro-movers-f...