fix ignored {label} on outputs

Author: Fil

README.md

@@ -2126,7 +2126,7 @@ The following aggregation methods are supported:
 * *y1* - the lower bound of the bin’s *y* extent (when binning on *y*)
 * *y2* - the upper bound of the bin’s *y* extent (when binning on *y*)
 * a function to be passed the array of values for each bin and the extent of the bin
-* an object with a *reduce* method, and optionally a *scope*
+* an object with a *reduce* method, and optionally a *scope* and a *label*
 
 In the last case, the *reduce* method is repeatedly passed three arguments: the index for each bin (an array of integers), the input channel’s array of values, and the extent of the bin (an object {x1, x2, y1, y2}); it must then return the corresponding aggregate value for the bin. If the reducer object’s *scope* is “data”, then the *reduce* method is first invoked for the full data; the return value of the *reduce* method is then made available as a third argument (making the extent the fourth argument). Similarly if the *scope* is “facet”, then the *reduce* method is invoked for each facet, and the resulting reduce value is made available while reducing the facet’s bins. (This optional *scope* is used by the *proportion* and *proportion-facet* reducers.)
 
@@ -2277,7 +2277,7 @@ The following aggregation methods are supported:
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
 * a function - passed the array of values for each group
-* an object with a *reduce* method, an optionally a *scope*
+* an object with a *reduce* method, an optionally a *scope* and a *label*
 
 In the last case, the *reduce* method is repeatedly passed two arguments: the index for each group (an array of integers), and the input channel’s array of values; it must then return the corresponding aggregate value for the group. If the reducer object’s *scope* is “data”, then the *reduce* method is first invoked for the full data; the return value of the *reduce* method is then made available as a third argument. Similarly if the *scope* is “facet”, then the *reduce* method is invoked for each facet, and the resulting reduce value is made available while reducing the facet’s groups. (This optional *scope* is used by the *proportion* and *proportion-facet* reducers.)
 

src/transforms/group.js

@@ -178,9 +178,10 @@ export function maybeOutputs(outputs, inputs, asOutput = maybeOutput) {
 
 export function maybeOutput(name, reduce, inputs, asEvaluator = maybeEvaluator) {
   let scale; // optional per-channel scale override
+  let {label} = reduce;
   if (isObject(reduce) && typeof reduce.reduce !== "function") (scale = reduce.scale), (reduce = reduce.reduce);
   const evaluator = asEvaluator(name, reduce, inputs);
-  const [output, setOutput] = column(evaluator.label);
+  const [output, setOutput] = column(label !== undefined ? label : evaluator.label);
   let O;
   return {
     name,

test/output/reducerGroupFunction.svg

@@ -220,7 +220,7 @@ export async function reducerGroupFunction() {
         penguins,
         Plot.groupX(
           {
-            y: "count",
+            y: {reduce: "count", label: "# of specimens"},
             fill: (values) => d3.mode(values)
           },
           {x: "species", fill: "island", fy: "sex"}