text font-size as an unscaled numerical channel

[Fil]

Interestingly, the result is quite smaller than a circle defined with the same accessor. So it seems that we need to introduce fontSizeAdjust (which isn't much supported as a css property https://caniuse.com/font-size-adjust), and rescale the resulting channel values in the font-size attribute. See for example https://observablehq.com/d/f5698ebfc65d2b5c#FSA

closes #296

Plot.plot({
  style: { background: "#fffbeb" },
  r: {
   range: [9, 25]
  },
  x: {
    label: "Birth Rate (per 1000 people) →"
  },
  y: {
    type: "sqrt",
    label: "↑ Death Rate (per 1000 people)"
  },
  color: {
    domain,
    range: color.range()
  },
  marks: [
    Plot.dot(data, {
      x: "birthRate", y: "deathRate", r: "population", stroke: "population", strokeWidth: 0.5,
      fill: asText ? null : "population",
    }),
    asText ? Plot.text(data, {
      x: "birthRate", y: "deathRate", fill: "population",
      text: "code", fontFamily: "Yanone Kaffeesatz", fontWeight: 700,
      fontSize: "population",
      fontSizeAdjust: .5
    })
      : [],
    Plot.ruleX([7]),
    Plot.ruleY([2.01])
  ],
  width: 954,
  height: 600
})

[mbostock]

Do you prefer this to a fontSize scale where you can set the range separately? Or perhaps no scale at all, and simply using the literal value similar to how we handle the rotate option? That feels like it might make the most sense for me, since font size is inherently fiddly and I don’t think we’d try to make a legend for it. But maybe it’s inconsistent?

Related, I’ve been thinking we need a strokeWidth scale or similar so that a line or link’s stroke width can be used as an encoding. That I think we would want a scale for, and we would also probably want a legend.

[Fil]

In the example of the image it makes sense to bind it to the r scale, but I agree it might be a rare case in which you would compute the scale outside the plot.

strokeWidth on links is definitely something that needs a scale. Not so sure about the legend, but if we follow #236 / #299 we can add as many legends as we want as plugins.

[mbostock]

Binding to the r scale feels surprising to me. I think we should probably not use a scaled channel here and have people compute font sizes manually. Then we won’t need the fontSizeAdjust property also.

[Fil]

Agreed, I'll update this accordingly.

[Fil]

Discussion loosely connected to #56 (comment)

[Fil]

ready