diff --git a/src/facet.js b/src/facet.js
index fc26316aa8..9320b99e38 100644
--- a/src/facet.js
+++ b/src/facet.js
@@ -1,7 +1,4 @@
-import {column, maybeColorChannel, maybeNumberChannel, slice, valueof} from "./options.js";
-import {maybeSymbolChannel} from "./symbols.js";
-import {maybeFontSizeChannel} from "./marks/text.js";
-import {maybePathChannel} from "./marks/image.js";
+import {column, slice, valueof} from "./options.js";
 
 export function facetReindex(facets, n) {
   if (facets.length === 1) return {facets};
@@ -54,44 +51,46 @@ export function maybeExpand(X, plan) {
 
 // Iterate over the options and pull out any that represent columns of values.
 const knownChannels = [
-  ["x"],
-  ["x1"],
-  ["x2"],
-  ["y"],
-  ["y1"],
-  ["y2"],
-  ["z"],
-  ["ariaLabel"],
-  ["href"],
-  ["title"],
-  ["fill", (value) => maybeColorChannel(value)[0]],
-  ["stroke", (value) => maybeColorChannel(value)[0]],
-  ["fillOpacity", (value) => maybeNumberChannel(value)[0]],
-  ["strokeOpacity", (value) => maybeNumberChannel(value)[0]],
-  ["opacity", (value) => maybeNumberChannel(value)[0]],
-  ["strokeWidth", (value) => maybeNumberChannel(value)[0]],
-  ["symbol", (value) => maybeSymbolChannel(value)[0]], // dot
-  ["r", (value) => maybeNumberChannel(value)[0]], // dot
-  ["rotate", (value) => maybeNumberChannel(value)[0]], // dot, text
-  ["fontSize", (value) => maybeFontSizeChannel(value)[0]], // text
-  ["text"], // text
-  ["length", (value) => maybeNumberChannel(value)[0]], // vector
-  ["width", (value) => maybeNumberChannel(value)[0]], // image
-  ["height", (value) => maybeNumberChannel(value)[0]], // image
-  ["src", (value) => maybePathChannel(value)[0]], // image
-  ["weight", (value) => maybeNumberChannel(value)[0]] // density
+  "x",
+  "x1",
+  "x2",
+  "y",
+  "y1",
+  "y2",
+  "z",
+  "ariaLabel",
+  "href",
+  "title",
+  "fill",
+  "stroke",
+  "fillOpacity",
+  "strokeOpacity",
+  "opacity",
+  "strokeWidth",
+  "symbol", // dot
+  "r", // dot
+  "rotate", // dot, text
+  "fontSize", // text
+  "text", // text
+  "length", // vector
+  "width", // image
+  "height", // image
+  "src", // image
+  "weight" // density
 ];
 
-export function maybeExpandOutputs(options) {
-  const other = {};
-  const outputs = [];
-  for (const [name, test = (value) => value] of knownChannels) {
-    const value = test(options[name]);
+export function maybeExpandChannels(options) {
+  const channels = {};
+  const [{transform: plan}, setPlan] = column();
+  for (const name of knownChannels) {
+    let value = options[name];
     if (value != null) {
-      const [V, setV] = column(value);
-      other[name] = V;
-      outputs.push((data, plan) => setV(maybeExpand(valueof(data, value), plan)));
+      if (value.definition) continue; // already planned
+      channels[name] = {
+        definition: value,
+        transform: (data) => maybeExpand(valueof(data, value), plan())
+      };
     }
   }
-  return [other, outputs];
+  return [channels, setPlan];
 }
diff --git a/src/marks/image.js b/src/marks/image.js
index 3eaef552d3..dea489736c 100644
--- a/src/marks/image.js
+++ b/src/marks/image.js
@@ -34,8 +34,13 @@ function isUrl(string) {
 
 // Disambiguates a constant src definition from a channel. A path or URL string
 // is assumed to be a constant; any other string is assumed to be a field name.
-export function maybePathChannel(value) {
-  return typeof value === "string" && (isPath(value) || isUrl(value)) ? [undefined, value] : [value, undefined];
+function maybePathChannel(value) {
+  if (typeof value === "string" && (isPath(value) || isUrl(value))) return [undefined, value];
+  if (value && value.definition) {
+    const [, f] = maybePathChannel(value.definition);
+    if (f !== undefined) return [undefined, f];
+  }
+  return [value, undefined];
 }
 
 export class Image extends Mark {
diff --git a/src/marks/text.js b/src/marks/text.js
index ccd5ec7fa7..d634f58b55 100644
--- a/src/marks/text.js
+++ b/src/marks/text.js
@@ -241,8 +241,12 @@ const fontSizes = new Set([
 // - string <length>: e.g., "12px"
 // - string <percentage>: e.g., "80%"
 // Anything else is assumed to be a channel definition.
-export function maybeFontSizeChannel(fontSize) {
+function maybeFontSizeChannel(fontSize) {
   if (fontSize == null || typeof fontSize === "number") return [undefined, fontSize];
+  if (fontSize && fontSize.definition) {
+    const [, f] = maybeFontSizeChannel(fontSize.definition);
+    if (f !== undefined) return [undefined, f];
+  }
   if (typeof fontSize !== "string") return [fontSize, undefined];
   fontSize = fontSize.trim().toLowerCase();
   return fontSizes.has(fontSize) || /^[+-]?\d*\.?\d+(e[+-]?\d+)?(\w*|%)$/.test(fontSize)
diff --git a/src/options.js b/src/options.js
index 94ca533c1a..1d8437ef55 100644
--- a/src/options.js
+++ b/src/options.js
@@ -71,6 +71,7 @@ export function percentile(reduce) {
 // CSS color, use an accessor (d => d.red) instead.
 export function maybeColorChannel(value, defaultValue) {
   if (value === undefined) value = defaultValue;
+  if (value && value.definition && isColor(value.definition)) return [undefined, value.definition];
   return value === null ? [undefined, "none"] : isColor(value) ? [undefined, value] : [value, undefined];
 }
 
@@ -78,6 +79,7 @@ export function maybeColorChannel(value, defaultValue) {
 // indicating a constant, and otherwise assumes that it’s a channel value.
 export function maybeNumberChannel(value, defaultValue) {
   if (value === undefined) value = defaultValue;
+  if (value && value.definition && typeof value.definition === "number") return [undefined, value.definition];
   return value === null || typeof value === "number" ? [undefined, value] : [value, undefined];
 }
 
@@ -252,7 +254,15 @@ export function maybeColumn(source) {
 }
 
 export function labelof(value, defaultValue) {
-  return typeof value === "string" ? value : value && value.label !== undefined ? value.label : defaultValue;
+  return typeof value === "string"
+    ? value
+    : value
+    ? value.label !== undefined
+      ? value.label
+      : typeof value.definition === "string"
+      ? value.definition
+      : defaultValue
+    : defaultValue;
 }
 
 // Assuming that both x1 and x2 and lazy columns (per above), this derives a new
diff --git a/src/symbols.js b/src/symbols.js
index 8c02b900f9..9b3526b65c 100644
--- a/src/symbols.js
+++ b/src/symbols.js
@@ -55,6 +55,7 @@ export function maybeSymbol(symbol) {
 
 export function maybeSymbolChannel(symbol) {
   if (symbol == null || isSymbolObject(symbol)) return [undefined, symbol];
+  if (symbol && symbol.definition && isSymbolObject(symbol.definition)) return [undefined, symbol.definition];
   if (typeof symbol === "string") {
     const value = symbols.get(`${symbol}`.toLowerCase());
     if (value) return [undefined, value];
diff --git a/src/transforms/map.js b/src/transforms/map.js
index 1cf2bc21a0..9d0661e1ed 100644
--- a/src/transforms/map.js
+++ b/src/transforms/map.js
@@ -1,7 +1,7 @@
 import {count, group, rank} from "d3";
 import {maybeZ, take, valueof, maybeInput, column} from "../options.js";
 import {basic} from "./basic.js";
-import {maybeExpand, facetReindex, maybeExpandOutputs} from "../facet.js";
+import {maybeExpand, facetReindex, maybeExpandChannels} from "../facet.js";
 
 /**
  * ```js
@@ -57,7 +57,7 @@ export function map(outputs = {}, options = {}) {
     const [output, setOutput] = column(input);
     return {key, input, output, setOutput, map: maybeMap(map)};
   });
-  const [other, facetOutputs] = maybeExpandOutputs(options); // TODO wrap outputs in facetReindex
+  const [other, setPlan] = maybeExpandChannels(options);
   return {
     ...basic(options, (data, facets) => {
       let plan;
@@ -65,7 +65,7 @@ export function map(outputs = {}, options = {}) {
       const Z = maybeExpand(valueof(data, z), plan);
       const X = channels.map(({input}) => maybeExpand(valueof(data, input), plan));
       const MX = channels.map(({setOutput}) => setOutput(new Array(plan ? plan.length : data.length)));
-      for (const o of facetOutputs) o(data, plan); // expand any extra channels
+      setPlan(plan); // expand any extra channels
       for (const facet of facets) {
         for (const I of Z ? group(facet, (i) => Z[i]).values() : [facet]) {
           channels.forEach(({map}, i) => map.map(I, X[i], MX[i]));
diff --git a/src/transforms/stack.js b/src/transforms/stack.js
index 1059d625c4..851ed4d0fa 100644
--- a/src/transforms/stack.js
+++ b/src/transforms/stack.js
@@ -1,6 +1,6 @@
 import {InternMap, cumsum, group, groupSort, greatest, max, min, rollup, sum} from "d3";
 import {ascendingDefined} from "../defined.js";
-import {maybeExpand, facetReindex, maybeExpandOutputs} from "../facet.js";
+import {maybeExpand, facetReindex, maybeExpandChannels} from "../facet.js";
 import {field, column, mid, range, valueof, one} from "../options.js";
 import {maybeColumn, maybeZ, maybeZero} from "../options.js";
 import {basic} from "./basic.js";
@@ -147,7 +147,7 @@ function stack(x, y = one, ky, {offset, order, reverse}, options) {
   const [Y2, setY2] = column(y);
   offset = maybeOffset(offset);
   order = maybeOrder(order, offset, ky);
-  const [other, outputs] = maybeExpandOutputs(options); // TODO wrap outputs in facetReindex
+  const [other, setPlan] = maybeExpandChannels(options);
   return [
     basic(options, (data, facets) => {
       let plan;
@@ -159,8 +159,8 @@ function stack(x, y = one, ky, {offset, order, reverse}, options) {
       const Y = maybeExpand(YS, plan);
       const Z = maybeExpand(ZS, plan);
       const O = order && maybeExpand(order(data, XS, YS, ZS), plan);
+      setPlan(plan); // expand any extra channels
       const n = plan ? plan.length : data.length;
-      for (const o of outputs) o(data, plan); // expand any extra channels
       const Y1 = setY1(new Float64Array(n));
       const Y2 = setY2(new Float64Array(n));
       const facetstacks = [];