« December 2019 | Main

Bubble charts, ratios and proportionality

A recent article in the Wall Street Journal about a challenger to the dominant weedkiller, Roundup, contains a nice selection of graphics. (Dicamba is the up-and-comer.)

Wsj_roundup_img1


The change in usage of three brands of weedkillers is rendered as a small-multiples of choropleth maps. This graphic displays geographical and time changes simultaneously.

The staircase chart shows weeds have become resistant to Roundup over time. This is considered a weakness in the Roundup business.

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In this post, my focus is on the chart at the bottom, which shows complaints about Dicamba by state in 2019. This is a bubble chart, with the bubbles sorted along the horizontal axis by the acreage of farmland by state.

Wsj_roundup_img2

Below left is a more standard version of such a chart, in which the bubbles are allowed to overlap. (I only included the bubbles that were labeled in the original chart).

Redo_roundupwsj0

The WSJ’s twist is to use the vertical spacing to avoid overlapping bubbles. The vertical axis serves a design perogative and does not encode data.  

I’m going to stick with the more traditional overlapping bubbles here – I’m getting to a different matter.

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The question being addressed by this chart is: which states have the most serious Dicamba problem, as revealed by the frequency of complaints? The designer recognizes that the amount of farmland matters. One should expect the more acres, the more complaints.

Let's consider computing directly the number of complaints per million acres.

The resulting chart (shown below right) – while retaining the design – gives a wholly different feeling. Arkansas now owns the largest bubble even though it has the least acreage among the included states. The huge Illinois bubble is still large but is no longer a loner.

Redo_dicambacomplaints1

Now return to the original design for a moment (the chart on the left). In theory, this should work in the following manner: if complaints grow purely as a function of acreage, then the bubbles should grow proportionally from left to right. The trouble is that proportional areas are not as easily detected as proportional lengths.

The pair of charts below depict made-up data in which all states have 30 complaints for each million acres of farmland. It’s not intuitive that the bubbles on the left chart are growing proportionally.

Redo_dicambacomplaints2

Now if you look at the right chart, which shows the relative metric of complaints per million acres, it’s impossible not to notice that all bubbles are the same size.


Taking small steps to bring out the message

Happy new year! Good luck and best wishes!

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We'll start 2020 with something lighter. On a recent flight, I saw a chart in The Economist that shows the proportion of operating income derived from overseas markets by major grocery chains - the headline said that some of these chains are withdrawing from international markets.

Econ_internationalgroceries_sm

The designer used one color for each grocery chain, and two shades within each color. The legend describes the shades as "total" and "of which: overseas". As with all stacked bar charts, it's a bit confusing where to find the data. The "total" is actually the entire bar, not just the darker shaded part. The darker shaded part is better labeled "home market" as shown below:

Redo_econgroceriesintl_1

The designer's instinct to bring out the importance of international markets to each company's income is well placed. A second small edit helps: plot the international income amounts first, so they line up with the vertical zero axis. Like this:

Redo_econgroceriesintl_2

This is essentially the same chart. The order of international and home market is reversed. I also reversed the shading, so that the international share of income is displayed darker. This shading draws the readers' attention to the key message of the chart.

A stacked bar chart of the absolute dollar amounts is not ideal for showing proportions, because each bar is a different length. Sometimes, plotting relative values summing to 100% for each company may work better.

As it stands, the chart above calls attention to a different message: that Walmart dwarfs the other three global chains. Just the international income of Walmart is larger than the total income of Costco.

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Please comment below or write me directly if you have ideas for this blog as we enter a new decade. What do you want to see more of? less of?