Area chart is not the solution
Feb 14, 2023
A reader left a link to a Wiki chart, which is ghastly:
This chart concerns the trend of relative proportions of House representatives in the U.S. Congress by state, and can be found at this Wikipedia entry. The U.S. House is composed of Representatives, and the number of representatives is roughly proportional to each state's population. This scheme actually gives small states disporportional representation, since the lowest number of representatives is 1 while the total number of representatives is fixed at 435.
We can do a quick calculation: 1/435 = 0.23% so any state that has less than 0.23% of the population is over-represented in the House. Alaska, Vermont and Wyoming are all close to that level. The primary way in which small states get larger representation is via the Senate, which sits two senators per state no matter the size. (If you've wondered about Nate Silver's website: 435 Representatives + 100 Senators + 3 for DC = 538 electoral votes for U.S. Presidental elections.)
So many things have gone wrong with this chart. There are 50 colors for 50 states. The legend arranges the states by the appropriate metric (good) but in ascending order (bad). This is a stacked area chart, which makes it very hard to figure out the values other than the few at the bottom of the chart.
A nice way to plot this data is a tile map with line charts. I found a nice example that my friend Xan put together in 2018:
A tile map is a conceptual representation of the U.S. map in which each state is represented by equal-sized squares. The coordinates of the states are distorted in order to line up the tiles. A tile map is a small-multiples setup in which each square contains a chart of the same design to faciliate inter-state comparisons.
In the above map, Xan also takes advantage of the foregrounding concept. Each chart actually contains all 50 lines for every state, all shown in gray while the line for the specific state is bolded and shown in red.
A chart with 50 lines looks very different from one with 50 areas stacked on each other. California, the most populous state, has 12% of the total population so the line chart has 50 lines that will look like spaghetti. Thus, the fore/backgrounding is important to make sure it's readable.
I suspect that the designer chose a stacked area chart because the line chart looked like spaghetti. But that's the wrong solution. While the lines no longer overlap each other, it is a real challenge to figure out the state-level trends - one has to focus on the heights of the areas, rather than the boundary lines.
[P.S. 2/27/2023] As we like to say, a picture is worth a thousand words. Twitter reader with the handle LHZGJG made the tile map I described above. It looks like this:
You can pick out the states with the key changes really fast. California, Texas, Florida on the upswing, and New York, Pennsylvania going down. I like the fact that the state names are spelled out. Little tweaks are possible but this is a great starting point. Thanks LHZGJG! ]
Another article to make one think for a moment.... but this time my thought diverge from yours concerning the tile map. In fact, following your comments (that all 50 traces are replicated with the current state in bold red), I'd expect that the lines in the background of each tile would look exceedingly similar if the same 50 traces were present (and there would be nothing special about the CA plot despite the large relative population).
Instead, it appears that each chart only contains 15 lines - one for each flu season in the state. This allows the shape of the background traces to change subtly from state to state - indicating for example that MA and NY typically have a much longer flu season than Kansas or New Mexico, and that flu season in DC and Hawaii tends to be less severe than most other regions.
Posted by: Adam | Feb 17, 2023 at 11:02 AM
Adam: The beauty of the Twitter community is someone answered the call. I added the realized tile map above. Can't vouch for the data but it works!
Posted by: Kaiser | Feb 17, 2023 at 02:03 PM