Should facets work on a Map?

[Fil]

If the data is a Map (for example obtained with d3.rollup), faceting doesn't work. Documentation or bug?

data = d3.rollup(…)

Plot.plot({
  facet: {
    data,
    x: "0"
  },
  marks: [Plot.barY(data, { x:"0", y: "1" })]
})

https://observablehq.com/d/a3de0aa6815cab5c

plot/src/facet.js

Line 41 in 9eeafaa

const markFacets = mark.data === this.data ? facetsIndex : undefined;

[mbostock]

This is a bug caused by the greedy arrayify here:

plot/src/mark.js

Line 11 in 8d308d0

this.data = arrayify(data);

[Fil]

Removing it doesn't break any test.

diff --git a/src/mark.js b/src/mark.js
index 504f93b..5b2a611 100644
--- a/src/mark.js
+++ b/src/mark.js
@@ -8,7 +8,7 @@ const objectToString = Object.prototype.toString;
 export class Mark {
   constructor(data, channels = [], options = {}) {
     const names = new Set();
-    this.data = arrayify(data);
+    this.data = data;
     this.transform = maybeTransform(options);
     this.channels = channels.filter(channel => {
       const {name, value, optional} = channel;

maybe for safety we can add it later, before we pass it to this.transform (but I doubt it's necessary?):

@@ -29,7 +29,7 @@ export class Mark {
     });
   }
   initialize(facets) {
-    let data = this.data;
+    let data = arrayify(this.data);
     let index = facets === undefined && data != null ? range(data) : facets;
     if (data !== undefined && this.transform !== undefined) {
       if (facets === undefined) index = index.length ? [index] : [];