This network visualization caught my eye on Twitter so I just had to investigate. It displays the volume of food imports and exports between states in 2007. They do several nice things in this plot: use both color and position on the circle to indicate total volume for a state, rather than just naively plotting in alphabetical order (the Austria First! principle), use line width to indicate volume of trade between states, and use the line colors to indicate whether the trade is incoming or outgoing.
The paper that this plot comes from is a straightforward mathematical analysis of the food commodity trade in the U.S. The paper is sterile in the best way possible: they don’t make any claims about what the properties of the network mean beyond the mathematical estimates. There are no modeling assumptions or p-values, just observations about the way food moves throughout the nation. They don’t overstate the importance of their findings. I like it.
They bring up two interesting points about the strength (volume of trade in and out) of the nodes (trade regions):
- Distribution of node neighbors: The distribution of global trade for all commodities is scale-free, meaning that the proportion of trade centers with greater than k trade neighbors decays approximately like (1/k)^2. This implies that there are many nodes with a few neighbors and a few nodes with many neighbors. Instead, the distribution of food trade in the U.S. is approximately normal, meaning that there are many nodes with a moderate number of trade neighbors and few nodes with lots or few neighbors. This distribution is more characteristic of a social network.
- Node strength rankings: The places with greatest inward flow strength include New Orleans-Metairi-Bogalusa, Texas, Los Angeles-Long Beach-Riverside, and Chicago-Naperville-Michigan City. That’s presumably because they’re hubs for railroads and ports, so lots of things get sent there and leave the country rather than go to other states. Conversely, the places with greatest outward flow strength include Iowa, Illinois, Missouri, and Nebraska. They probably send out enormous volumes of corn and soy, America’s favorite cash crops.
There’s nothing particularly surprising about these results, but it’s nice to see them presented in a clear way supported by the numbers.