nicer label for extra channels

Author: mbostock

src/mark.js

@@ -150,7 +150,7 @@ export function composeRender(r1, r2) {
 function maybeChannels(channels) {
   return Object.fromEntries(
     Object.entries(maybeNamed(channels)).map(([name, channel]) => {
-      channel = maybeValue(channel);
+      channel = typeof channel === "string" ? {value: channel, label: name} : maybeValue(channel); // for shorthand extra channels, use name as label
       if (channel.filter === undefined && channel.scale == null) channel = {...channel, filter: null};
       return [name, channel];
     })

src/marks/tip.js

@@ -116,15 +116,16 @@ export class Tip extends Mark {
     const widthof = monospace ? monospaceWidth : defaultWidth;
     const ee = widthof(ellipsis);
 
-    // Initialize shorthand formats. TODO Don’t mutate.
+    // Promote shorthand string formats to functions. TODO Don’t mutate.
     for (const key in this.format) {
       const format = this.format[key];
       if (typeof format === "string" && key in sources) {
         this.format[key] = (isTemporal(sources[key].value) ? utcFormat : numberFormat)(format);
       }
     }
 
-    // Initialize default formats for the facet scales. TODO Don’t mutate.
+    // Borrow the scale tick format for facet channels; this is generally better
+    // than the default format (and safe for ordinal scales). TODO Don’t mutate.
     if (index.fi != null) {
       const {fx, fy} = scales;
       if (fx && this.format.fx === undefined) this.format.fx = inferTickFormat(fx, fx.domain());