The reckless practice of eyeballing trend lines

MSN showed this chart claiming a huge increase in the number of British children who believe they are born the wrong gender.

The graph has a number of defects, starting with drawing a red line that clearly isn’t the trend in the data.

To find the trend line, we have to draw a line that is closest to the top of every column. The true trend line is closer to the blue line drawn below:

The red line moves up one unit roughly every three years while the blue line does so every four years.

Notice the dramatic jump in the last column of the chart. The observed trend is not a straight line, and therefore it is not appropriate to force a straight-line model. Instead, it makes more sense to divide the time line into three periods, with different rates of change.

Most of the growth during this 10 year period occurred in the last year, and one should check the data, and also check to see if any accounting criterion changed that might explain this large unexpected jump.

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The other curiosity about this chart is the scale of the vertical axis. Nowhere on the chart does it say which metric of gender dysphoria it is depicting. The title suggests they are counting the number of diagnoses but the axis labels that range from one to five point to some other metric.

From the article, we learn that annual number of gender dysphoria diagnoses was about 10,000 in 2021, and that is encoded as 4.5 in the column chart. The sub-header of the chart indicates that the unit is number per 1,000 people. Ten thousand diagnoses divided by the population size of under 18 x 1,000 = 4.5. This implies there were roughly 2.2 million people under 18 in the U.K. in 2021.

But according to these official statistics (link), there were about 13 million people aged 0-18 in just England and Wales in mid-2022, which is not in the right range. From a dataviz perspective, the designer needs to explain what the values on the vertical axes represent. Right now, I have no idea what it means.

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Using the Trifecta Checkup framework, we say that the question addressed by the chart is clear but there are problems relating to data encoding as well as the trend-line visual.

Comments

[Art]

This is the junkiest chart you've posted. "Number goes up" is the only thing you can take away. It insults the audience.