Channel label option

[Fil]

In this discussion we realize that to specify a channel label (for the interactive tips #1612), we need either a {transform, label} value, or an arcane combination of an accessor function with a label property:

channels: {year: Object.assign((d) => d.year, {label: "My custom year label"})} 

This is because labelof only reads the value.

It would be nice to be allowed to write instead:

channels: {year: {value: "year", label: "My custom year label"}}

Note that this is not strictly necessary for the given use case, since we can also name the channel as we want (although not with a string which has precedence over the channel’s key name):

channels: {["My custom year label"]: d => d.year}

(maybe related to #1364)

[Fil]

potential patch (not yet tested):

diff --git a/src/channel.js b/src/channel.js
index 38edcc3e..019efc26 100644
--- a/src/channel.js
+++ b/src/channel.js
@@ -5,13 +5,13 @@ import {registry} from "./scales/index.js";
 import {isSymbol, maybeSymbol} from "./symbol.js";
 import {maybeReduce} from "./transforms/group.js";

-export function createChannel(data, {scale, type, value, filter, hint}, name) {
+export function createChannel(data, {scale, type, value, filter, hint, label}, name) {
   if (hint === undefined && typeof value?.transform === "function") hint = value.hint;
   return inferChannelScale(name, {
     scale,
     type,
     value: valueof(data, value),
-    label: labelof(value),
+    label: label === undefined ? labelof(value) : label,
     filter,
     hint
   });