Check out Manifest Destiny. This is a great way to visualize and teach about the expansion of the United States through time. Plus the site is very slick and easy to use.
From the web site: Manifest Destiny tells the story of the United States in 141 maps from the Declaration of Independence to the present.
Hover over the Legend and the legend shows up (see above).
Hover over a thumbnail of a map and see the date (below).
Click on a map and get more information. You can also highlight the changes on the map (below)
Great work by data visualization consultant, Michael Porath. Thank you, Aprecido Leite, for the tweet.
