nullish and function marks

src/facet.js

@@ -1,6 +1,6 @@
 import {cross, difference, groups, InternMap} from "d3";
 import {create} from "d3";
-import {Mark, first, second} from "./mark.js";
+import {Mark, first, second, markify} from "./mark.js";
 import {applyScales} from "./scales.js";
 import {filterStyles} from "./style.js";
 
@@ -21,7 +21,7 @@ class Facet extends Mark {
       ],
       options
     );
-    this.marks = marks.flat(Infinity);
+    this.marks = marks.flat(Infinity).map(markify);
     // The following fields are set by initialize:
     this.marksChannels = undefined; // array of mark channels
     this.marksIndex = undefined; // array of mark indexes (for non-faceted marks)

src/mark.js

@@ -271,3 +271,17 @@ export function isTemporal(values) {
     return value instanceof Date;
   }
 }
+
+export function markify(mark) {
+  return mark instanceof Mark ? mark : new Render(mark);
+}
+
+class Render extends Mark {
+  constructor(render) {
+    super();
+    if (render == null) return;
+    if (typeof render !== "function") throw new TypeError("invalid mark");
+    this.render = render;
+  }
+  render() {}
+}

src/plot.js

@@ -1,6 +1,7 @@
 import {create} from "d3";
 import {Axes, autoAxisTicks, autoAxisLabels} from "./axes.js";
 import {facets} from "./facet.js";
+import {markify} from "./mark.js";
 import {Scales, autoScaleRange, applyScales} from "./scales.js";
 import {filterStyles, offset} from "./style.js";
 
@@ -15,7 +16,7 @@ export function plot(options = {}) {
   }
 
   // Flatten any nested marks.
-  const marks = options.marks === undefined ? [] : options.marks.flat(Infinity);
+  const marks = options.marks === undefined ? [] : options.marks.flat(Infinity).map(markify);
 
   // A Map from Mark instance to an object of named channel values.
   const markChannels = new Map();

test/plots/empty.js

@@ -1,4 +1,5 @@
 import * as Plot from "@observablehq/plot";
+import {svg} from "htl";
 
 export default async function() {
   return Plot.plot({
@@ -11,7 +12,12 @@ export default async function() {
       domain: [0, 1]
     },
     marks: [
-      Plot.frame()
+      Plot.frame(),
+      undefined,
+      null,
+      () => null,
+      () => undefined,
+      () => svg`<circle cx=50% cy=50% r=5 fill=green>`
     ]
   });
 }