## Chart without an axis

##### May 13, 2024

When it comes to global warming, most reports cite a single number such as an average temperature rise of Y degrees by year X. Most reports also claim the existence of a consensus within scientists. The Guardian presented the following chart that shows the spread of opinions amongst the experts.

Experts were asked how many degrees they expect average global temperature to increase by 2100. The estimates ranged from "below 1.5 degrees" to "5 degrees or more". The most popular answer was 2.5 degrees. Roughly three out of four respondents picked a number at 2.5 degrees or above. The distribution is close to symmetric around the middle.

***

What kind of chart is this?

It's a type of histogram, given that the horizontal axis shows binned ranges of temperature change while the vertical axis shows number of respondents (out of 380).

A (count) histogram typically encodes the count data in the vertical axis. Did you notice there isn't a vertical axis?

That's because the chart has an abnormal axis. Each of the 380 respondents is shown here as a cell. What looks like a "column" is actually two-dimensional. Each row of cells has 10 slots. To find out how many respondents chose the 2.5 celsius category, you count the number of rows and then the number of stray items on top. (It's 132.)

Only the top row of cells can be partially filled so the general shape of the distribution isn't affected much. However, the lack of axis labels makes it hard to learn the count of each column.

It's even harder to know the proportions of respondents, which should be the primary message of the chart. The proportion would have been possible to show if the maximum number of rows was set to 38. The maximum number of rows on the above chart is 22. Using 38 rows leads to a chart with a lot of white space as the tallest column (count of 132) is roughly 35% of the total response.

At the end, I'm not sure this variant of histogram beats the standard histogram.

"The distribution is close to symmetric around the middle."

One plausible hypothesis is that the survey gave respondents a set of discrete answers and most picked values near the middle. A similar histogram would probably be made for "number of five star Michelin restaurants in Bavaria" or "the total payroll of the 2024 Kansas City Chiefs" given a reasonable set of numeric categories and the exact same set of respondents. Indeed, one would probably see, given the subject matter expertise of climate scientists of NFL franchises, the same pattern were the Kansas City Chiefs' payroll centered on $150 million or $250 million (with increments of say, $20 million), likewise, 15 or 25 Michelin restaurants.

Posted by: Cody Custis | May 28, 2024 at 11:31 AM