fix missing facet data error (#1521)

Author: mbostock

src/plot.js

@@ -398,8 +398,8 @@ function maybeTopFacet(facet, options) {
   if (facet == null) return;
   const {x, y} = facet;
   if (x == null && y == null) return;
-  const data = arrayify(facet.data ?? x ?? y);
-  if (data === undefined) throw new Error(`missing facet data`);
+  const data = arrayify(facet.data);
+  if (data == null) throw new Error("missing facet data");
   const channels = {};
   if (x != null) channels.fx = createChannel(data, {value: x, scale: "fx"});
   if (y != null) channels.fy = createChannel(data, {value: y, scale: "fy"});

test/facet-test.js

@@ -31,3 +31,8 @@ it("mark data not parallel to facet data does not trigger a warning", async () =
     Plot.dot(data.slice(0, 1), {x: "x", y: "y", facet: undefined}).plot({facet: {data, x: "series"}})
   );
 });
+
+it("detects missing facet data", async () => {
+  assert.throws(() => Plot.barY([], {x: "x", y: "y"}).plot({facet: {x: "fx"}}), /missing facet data/);
+  assert.throws(() => Plot.barY([], {x: "x", y: "y"}).plot({facet: {data: null, x: "fx"}}), /missing facet data/);
+});