The scale’s transform should apply to the scale’s explicit domain, if any

In this example, it’s confusing that the domain is specified in the transformed space [-100, 100] instead of the pre-transformed space [0, 2]; everything else, including the ruleY is specified in the pre-transformed space. So, you want the domain to be specified in the same space so that you can add and remove the transform without needing to change anything else in the plot.

Plot.plot({
  y: {
    domain: [-100, 100],
    label: "↑ Close (%)",
    transform: (y) => (y - 1) * 100,
    tickFormat: "+d"
  },
  color: {
    legend: true
  },
  marks: [
    Plot.ruleX([frcb[i].Date]),
    Plot.ruleY([1]),
    Plot.lineY(stocks, Plot.normalizeY((Y) => Y[i], {x: "Date", y: "Close", stroke: "Symbol", tip: true})),
  ]
})

I think the reason we don’t do this now somewhat inadvertent: when the domain is not specified explicitly, it is derived from the channels, which already have the transform applied. So, we don’t want to apply the transform to the domain if the domain was derived from already-transformed channels; we only want to apply it when the domain was specified explicitly.

I don’t see an obvious way of making this change backwards compatible, since it changes the meaning of the domain option when the transform option is present. (We could introduce another option to control whether the transform applies to the domain, but it’ll still be confusing unless we change the default behavior.) So maybe this isn’t fixable, but I figure I would at least write this down in case others run into the same confusion.

Start with the y-scale configuration in the example and trace how an explicit domain differs from a domain derived from transformed channels. The desired behavior is for an explicit domain such as [0, 2] to receive the scale transform while derived domains retain their current behavior; assess the backward-compatibility concern before defining the change.