## Visualizing movements of people

##### Jan 06, 2014

Long-time reader Daniel L. sends in this chart illustrating a large data set of intra-state migration flows in the U.S. The original chart is at Vizynary by way of Daily Kos.

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There is no denying that this chart is beautiful to look at. But what is its message? That there are people migrating from and to every state? (assuming all fifty states are present)

Daily Kos describes how one can hover over any state to see its individual patterns. Something like this:

This is a great way, perhaps the only way, to consume the chart. Essentially, the reader is asked to generate a small-multiples panel of charts. The chart does a better job at showing the pairs of states between which people migrate than at showing the relative size of the flows. The size of the flows is coded in the width of the arcs. The widths are too similar to tell apart; and it doesn't help that no legend is provided.

The choice of color is curious. Each region of the country is its own color, in a "nominal" way. It is a design decision to emphasize regions.

Another decision is to hide information on the distances of the migrations. Evidently, the designer sacrificed that information in order to create the neat circular arrangement of states.

A shortcoming of this representation is one missing dimension: the direction of the flow. I'm not sure given any pair of states A and B, whether the net migration is into A or into B.

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I propose a solution using the map while preserving the interactive element of the original.

On this map, when you hover over a particular state, it highlights all other states for which there are migrations flows into or out of that state. For color, use a blue-white-red scheme with blue indicating net inflow, red indicating net outflow, and white for near-zero flows. Include a legend.

Another important decision for the designer is absolute versus relative scales. In an absolute scheme, you rank the entire set of flows for all pairs of states; obviously, the resulting colors would be influenced by the state populations. Alternatively, you rank the flow sizes within each state; in this case, the smaller states will feel exaggerated.

The map has the additional advantage of showing the approximate distance (and direction) moved, which, for me, is a useful piece of information.