>>Thanks. I like the way it shows popularity of stories and their movement since the last update. New stories, stories becoming more popular, and stories becoming less popular - you can see it all at a glance.
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>Yep. Is there anything that tells you the significance of the various colors?

Yes, the help button (life-saver icon). But in a nutshell;

Blue - The story is new in the News Map. It did not exist in the previous News Map. This is the starting colour for all squares.

Green - The story is becoming more popular and is moving up in the News Map rankings.

Red - The story is becoming less popular and moving down in the News Map rankings.

Grey - The story has not changed it's ranking within the News Map since the previous update.