How to get mean value within data of a facet?

[knutwannheden]

I am building a faceted strip plot chart in which I would like to display a red rule for the mean value within the facet. I found Plot.selectMinY() and Plot.selectMaxY(), but I don't understand how I can display the mean value. Tips would be most appreciated.

In the example below I've used Plot.selectMinY():

The code (published at https://observablehq.com/d/4a0159d26906f08d) looks like this:

Plot.plot({
  grid: true,
  inset: 10,
  height: 400,
  width: 800,
  facet: {
    data: ib_tr,
    x: "fraktion"
  },
  y: {
    nice: true
  },
  fx: {
    domain: ['G', 'S', 'GL', 'M-E', 'RL', 'V'], 
  },
  marks: [
    Plot.ruleY([0]),
    Plot.ruleY(ib_tr, Plot.selectMinY({y: "rating", stroke: "red"})),
    Plot.dot(ib_tr, Plot.dodgeX({
      y: "rating",
      r: 4,
      title: d =>`${d.parlamentarier}\nMandate: ${d.anzahl}`,
      href: d => `https://lobbywatch.ch/de/daten/parlamentarier/${d.parlamentarier_id}`,
      target: '_blank',
      fill: "fraktion",
      anchor: "middle",
      id: d => d.parlamentarier_id
    }))
  ],
  color: {
    legend: false,
    type: "categorical",
    domain: ['G', 'S', 'GL', 'M-E', 'RL', 'V'], 
    range: ["#85B50C", "#E4022D", "#A0A000", "#FF9100", "#3A8BC1", "#0A7228"]
  },
})

[knutwannheden]

I think I found a solution or at least a workaround based on the README.md in the GitHub repo:

    Plot.ruleY(ib_tr, Plot.select({
      y: (I, V) => {
        const median = d3.mean(I, i => V[i]);
        const i = d3.least(I, i => Math.abs(V[i] - median));
        return [i];
      }
    }, {y: "rating", stroke: "red"})),

But AFAICT this will give me the data point that is nearest the mean value, which wasn't exactly what I wanted, but I will use it as a workaround.

[mbostock]

You can use the groupZ transform:

Plot.ruleY(ib_tr, Plot.groupZ({y: "median"}, {y: "rating", stroke: "red"}))

[knutwannheden]

Thanks a lot. This looks exactly like what I wanted!

I still need to wrap my head around this z-channel...

[knutwannheden]

The facets documentation doesn't appear to mention the z channel. Where would I have found the hint for this?

[mbostock]

Here there’s no z-channel, so Plot.groupZ means “group everything”. It’s a bit confusing… 🤷 Normally the z-channel is used e.g. with a multi-series line chart where you want the lines to the same color.