Reading between the gridlines

Reader Jamie H. pointed me to the following chart in the Guardian (link), which originated from Spotify.

This chart is likely inspired by the Arctic ice cover chart discussed here last year (link):

Spotify calls its chart "the Coolness Spiral of Death" while the other one is called "Arctic Death Spiral".

The spiral chart has many problems, some of which I discussed in the post from last year. Just take a look at the headline, and then the black dotted spiral. Does the shape invoke the idea of rapid evolution, followed by maturation? Or try to figure out the amount of evolution between ages 18 and 30.

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Instead of the V corner of the Trifecta, I'd like to focus on the D corner today. When I look at charts, I'm always imagining the data behind the chart. Here are some questions to ponder:

• Given that Spotify was founded in 2006 (not quite 10 years ago), how are they able to discern someone's music taste from 14 through 48?
• The answer to the above question is they don't have a longitudinal view of anyone's music taste. They are comparing today's 14-year-old kid with today's 48-year-old adult. Under what assumptions would such an analysis yield the same outcome as a proper analysis that tracks the same people over time?
• If the phenomenon under study follows a predictable trend, there will be little difference between the two ways of looking at the data. For example, teeth in the average baby follow a certain sequence of emergence, first incisors at six months, and first molars at 14 months (according to Wikipedia). Observing John's teething at six months and David's at 14 months won't yield much difference from looking at John at six then 14 months. Does music taste evolve like human growth?
• Unfortunately, no. Imagine that a new genre of music suddenly erupts and it becomes popular among every generation of listeners. This causes the Spotify curve to shift towards the origin at all ages. However, if you take someone who is currently 30 years ol, the emergence of the new genre should affect his profile at age 30 but not anytime before. In fact, the new music creates a sharp shift at different locations of everyone's taste profile depending on one's age!
• Let's re-interpret the chart, and accept that each spoke in the wheel concerns a different cohort of people. So we are looking at generational differences. Is the Spotify audience representative of music listeners? Particularly, is each Spotify cohort representative of all listeners of that age?
• I find it unlikely since Spotify has that "cool" factor. It is probably more representative for younger age groups. Among older customers, there should be some bias. How does this affect the interpretation of the taste profile?
• If we find that one cohort differs from another cohort, it is important to establish that the gap is a generational difference and not due to the older age group being biased (self-selected) in some way.