add cairo's lines

content/snacks/cairo-discrete-line/index.md

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+title = 'Cairo Discrete Line'
+date = 2024-10-09T13:24:31-05:00
+draft = true
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+Line charts can be used (with caution) even when the X axis is neither interval or continuous.
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+We were discussing "rules" around line charts in class, and a student brought me a nice example. That shows the principles in action.
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+I hesitate to give "rules," especially in terms of chart types. But the guidance on "bar/dot vs. line" chart is pretty common. The principle is that connecting the dots implies the interpolation between the end points and emphasizes the slope; if interpolation and/or slope isn't meaningful, connecting the dots should be done with caution. It still may be useful (because it can help with connection).
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+(Using the NOIR levels of measurements: Rational, Interval, Ordinal, Nominal/Categorical).
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+So the "rules" suggest that line designs are good when the X axis is continuous and interval, acceptible if the X axis is interval, can be used with caution if it is ordered, and should be avoided for nominal categorical.
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+A student brought this example from Cairo's book:
+{{< rimage src="cairo-discrete-lines.png" width=400 caption="A line graph with a discrete axis." attr="Figure 5.17 of Cairo's *The Truthful Art*, p. 140." >}}
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+The student asked why connecting the dots was a valid choice (at first, he didn't realize it was ordinal).
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+1. It's Cairo and presented in his published work - so if he does flaunt a rule, he probably did it intentionally. 
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+2. The axis is definitely (at least) ordinal (life stages/generations in order). 
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+3. To emphasize that the lines are there to connect discrete things, there are dots. Notice that Cairo is using the edges for connection and making the discrete points very salient. 
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+4. I might argue it is an interval/continuous axis – if we think of it as “life stages” – it is totally sensible to think about being halfway between stage 1 and stage 2, or even the slope as "the rate of decrease in influence as we move between life stages".
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+One thing that strikes me in the image: my eye does follow the slopes. For example, the trend in the red line (Television) as we go from Xers, to Boomers, to Mature (3,4,5) is a pretty constant slope. Is this the wrong thing being easy to see? In this graph, I think that slopes could be meaningful - the rate at which things decrease as we move through life stages. It is not an unreasonable thing to think about (at least qualitatively).