Aligning the visual and the message to hot things up
Sep 25, 2024
The headline of this NBC News chart (link) tells readers that Phoenix (Arizona) has been very, very hot this year. It has over 120 days in which the average temperature exceeded 100F (38 C).
It's not obvious how extreme this situation is. To help readers, it would be useful to add some kind of reference points.
A couple of possibilities come to mind:
First, how many days are depicted in the chart? Since there is one cell for each day of the year, and the day of week is plotted down the vertical axis, we just need to count the number of columns. There are 38 columns, but the first column has one missing cell while the last column has only 3 cells. Thus, the number of days depicted is (36*7)+6+3 = 261. So, the average temperature in Phoenix exceeded 100F on about 46% of the days of the year thus far.
That sounds like a high number. For a better reference point, we'd also like to know the historical average. Is Phoenix just a very hot place? Is 2024 hotter than usual?
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Let's walk through how one reads the Phoenix "heatmap".
We already figured out that each column represents a week of the year, and each row shows a cross-section of a given day of week throughout the year.
The first column starts on a Monday because the first day of 2024 falls on a Monday. The last column ends on a Tuesday, which corresponds to Sept 17, 2024, the last day of data when this chart was created.
The columns are grouped into months, although such division is complicated by the fact that the number of days in a month (except for a leap month) isn't ever divisible by seven. The designer subtly inserted a thicker border between months. This feature allows readers to comment on the average temperature in a given month. It also lets readers learn quickly that we are two weeks and three days into September.
The color legend explains that temperature readings range from yellow (lower) to red (higher). The range of average daily temperatures during 2024 was 54-118F (12-48C). The color scale is progressive.
Given that 100F is used as a threshold to define "hot days," it makes sense to accentuate this in the visual presentation. For example:
Here, all days with maximum temperature at 100F or above have a red hue.