Visualizing the growth of Target and Walmart

Nathan at FlowingData has a new visualization displaying the opening of all Target stores since 1962 on a map of the United States.  This is a followup to a similar visualization on the growth of Walmart. (Target and Walmart are both department store chains in the US.) I like the use of the large-circle "explosions" to draw the eye to new data points over time. The use of semi-transparent dots to represent each store is also especially clever -- since most stores are clustered into local geographic areas, they reinforce each other to make a brighter color as each new dot os overlaid on existing dots.  The end result is almost like an intensity chart.  You can achieve a similar effect in R by using the alpha option (transparency percentage) to rgb when setting colors: a value near 0 makes a very translucent color, and a value near 1 makes your point almost solid.

It would be interesting to compare the growth of the two chains by animating both series on map at the same time.