## Messing with expectations

##### Jan 11, 2024

A co-worker sent me to the following map, found in Forbes:

It shows the amount of state tax surcharge per gallon of gas in the U.S. And it's got one of the most common issues found in choropleth maps - the color scheme runs opposite to reader expectations.

Typically, if we see a red-green color scale, we would expect red to represent large numbers and green, small numbers. This map reverses the typical setup: California, the state with the heftiest gas tax, is shown green.

I know, I know - if we apply the typical color scheme, California would bleed red, and it's a blue state, damn it.

The solution is to avoid the red color. Just don't use red or blue.

There is no need to use two colors either.

***

A few minor fixes. Given that all dollar amounts on the map are shown to two decimal places, the legend labels should also be shown to 2 decimal places, and with dollar signs.

The subtitle should read "Dollars per gallon" instead of "Cents per gallon". Alternatively, keep "Cents per gallon" but convert all data labels into cents.

Some of the states are missing data labels.

***

I recast this as a small-multiples by categorizing states into four subgroups.

With this change, one can almost justify using maps because there is sort of a spatial pattern.

You can follow this conversation by subscribing to the comment feed for this post.

I suspect the author was trying to use the "green" environmental definition by using green to show states with the highest penalty to gas usage. I do agree that there was no need for a two-color scale, but I think the overall decision for green is defensible, if maybe not the best.
I tend to think of green = good regardless of the direction/type of number (large/small/negative/positive) -- which in this case would be small from a reader/consumer point of view -- so it did have an unintended effect which the "cuteness" of using the "environmental green" does not do enough to justify.

>Typically, if we see a red-green color scale, we would expect red to represent large numbers and green, small numbers.

That's not exactly true, people often also assume that green = "good" numbers and red = "bad" numbers. IIRC, which colour scheme to use in case when the intended metric is generally understood as bad when is low was the most common point of discussion when I was doing visualisations for one data journalism project.

A: Thanks for the comment. Certainly, red and green are heavily loaded colors as you're confirming, and so, my recommendation is still to avoid them unless your field has an absolutely unmistakable convention in place.
Specifically, your point doesn't work on this example as having the highest surcharge is unlikely to be viewed as a "good" thing.

The comments to this entry are closed.