Multi-line format helper?

Should we expose a multi-line format helper? You can do it yourself but it’s a bit verbose:

tickFormat(x, i, X) {
  const format1 = (x) => x.toLocaleString("it", { day: "numeric" });
  const format2 = (x) => x.toLocaleString("it", { month: "short" });
  const f1 = format1(x);
  const f2 = format2(x);
  return i > 0 && f2 === format2(X[i - 1]) ? f1 : [f1, f2].join("\n");
}

Maybe:

tickFormat: Plot.formatConditional({ day: "numeric" }, { month: "short" })

But, you’d also need a way to pass in the locale, perhaps the template.

Start by reviewing the existing formatting API and the proposed Plot.formatConditional entry point. Compare the examples for repeated tick labels, multiple format specifications, and locale handling; done should mean a documented, agreed-upon API that supports the intended multi-line formatting behavior.