ignore null marks

src/facet.js

@@ -18,7 +18,7 @@ class Facet extends Mark {
       ],
       transform
     );
-    this.marks = marks;
+    this.marks = marks.filter(d => !!d);
     // 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/plot.js

@@ -34,6 +34,7 @@ export function plot(options = {}) {
   // Initialize the marks’ channels, indexing them by mark and scale as needed.
   // Also apply any scale transforms.
   for (const mark of marks) {
+    if (!mark) continue;
     if (markChannels.has(mark)) throw new Error("duplicate mark");
     const named = Object.create(null);
     const {index, channels} = mark.initialize();
@@ -93,6 +94,7 @@ export function plot(options = {}) {
       .text(`.plot text { white-space: pre; }`);
 
   for (const mark of marks) {
+    if (!mark) continue;
     const channels = markChannels.get(mark);
     const index = markIndex.get(mark);
     const node = mark.render(index, scales, channels, dimensions, axes);

test/marks/empty-test.js

@@ -0,0 +1,23 @@
+import * as Plot from "@observablehq/plot";
+import {JSDOM} from "jsdom";
+import tape from "tape-await";
+
+tape("empty marks are ignored", test => {
+  global.document = new JSDOM("").window.document;
+  test.equal(
+    Plot.plot({ marks: [undefined, 0, null, Plot.dotX([0])] }).innerHTML,
+    Plot.plot({ marks: [Plot.dotX([0])] }).innerHTML
+  );
+  const data = [1,2,3];
+  test.equal(
+    Plot.plot({
+      facet: { data, y: d => d },
+      marks: [ false, Plot.dotX(data) ]
+    }).innerHTML,
+    Plot.plot({
+      facet: { data, y: d => d },
+      marks: [ Plot.dotX(data) ]
+    }).innerHTML
+  );
+  delete global.document;
+});