Quick example of layering

The New York Times uses layering to place the Alabama tornadoes in context. (link)

Today's wide availability of detailed data allows designers to create dense data graphics like this:

The graphic shows the starting and ending locations and trajectory of each tornado, as well as the wind speeds (shown in color).

Too much data slows down our understanding of the visual message. The remedy is to subtract. Here is a second graphic that focuses only on the strongest tornadoes (graded 4 or 5 on a 5-point scale):

Another goal of the data visualization is to place in context the tornado that hit Beauregard:

The area around Beauregard is not typically visited by strong tornadoes. Also, the tornadoes were strong but there have been stronger ones.

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The designer unfolds the story in three stages. There are no knobs and sliders and arrows, and that's a beauty. It's usually not a good idea to make readers find the story themselves.