diff --git a/packages/mosaic/core/src/index.ts b/packages/mosaic/core/src/index.ts
index 3af377c8..a349b75b 100644
--- a/packages/mosaic/core/src/index.ts
+++ b/packages/mosaic/core/src/index.ts
@@ -31,6 +31,7 @@ export { isArrowTable } from './util/is-arrow-table.js';
 export { Synchronizer } from './util/synchronizer.js';
 export { throttle } from './util/throttle.js';
 export { toDataColumns } from './util/to-data-columns.js';
+export type { Arrayish, DataColumns } from './util/to-data-columns.js';
 export { queryFieldInfo } from './util/field-info.js';
 export { jsType } from './util/js-type.js';
 export { isActivatable } from './util/is-activatable.js';
diff --git a/packages/mosaic/core/src/util/to-data-columns.ts b/packages/mosaic/core/src/util/to-data-columns.ts
index 3f0861a6..8a7017c5 100644
--- a/packages/mosaic/core/src/util/to-data-columns.ts
+++ b/packages/mosaic/core/src/util/to-data-columns.ts
@@ -4,14 +4,14 @@ import type { Table } from '@uwdata/flechette';
 /**
  * An Array or TypedArray
  */
-type Arrayish = Array<unknown> | Int8Array | Uint8Array | Uint8ClampedArray
+export type Arrayish = Array<unknown> | Int8Array | Uint8Array | Uint8ClampedArray
   | Int16Array | Uint16Array | Int32Array | Uint32Array
   | Float32Array | Float64Array;
 
 /**
  * Data columns structure with either named columns or values array
  */
-type DataColumns =
+export type DataColumns =
   | { numRows: number; columns: Record<string, Arrayish> }
   | { numRows: number; values: Arrayish };
 
diff --git a/packages/vgplot/plot/src/interactors/Interval1D.js b/packages/vgplot/plot/src/interactors/Interval1D.js
index 50825feb..eb01f11f 100644
--- a/packages/vgplot/plot/src/interactors/Interval1D.js
+++ b/packages/vgplot/plot/src/interactors/Interval1D.js
@@ -1,3 +1,4 @@
+/** @import { InteractorMark } from '../marks/Mark.js' */
 import { clauseInterval } from '@uwdata/mosaic-core';
 import { ascending, min, max } from 'd3';
 import { brushGroups, brushX, brushY } from './util/brush.js';
@@ -11,6 +12,10 @@ import { sanitizeStyles } from './util/sanitize-styles.js';
  * @implements {Activatable}
  */
 export class Interval1D {
+  /**
+   * @param {InteractorMark} mark The mark to interact with.
+   * @param {*} options The interactor options.
+   */
   constructor(mark, {
     channel,
     selection,
diff --git a/packages/vgplot/plot/src/interactors/Nearest.js b/packages/vgplot/plot/src/interactors/Nearest.js
index 0880d3f2..56faee41 100644
--- a/packages/vgplot/plot/src/interactors/Nearest.js
+++ b/packages/vgplot/plot/src/interactors/Nearest.js
@@ -1,4 +1,5 @@
 /** @import { ClauseSource } from '@uwdata/mosaic-core' */
+/** @import { InteractorMark } from '../marks/Mark.js' */
 import { clauseList, clausePoint, clausePoints, isSelection } from '@uwdata/mosaic-core';
 import { select, pointer, min } from 'd3';
 import { getField } from './util/get-field.js';
@@ -8,6 +9,10 @@ import { getField } from './util/get-field.js';
  * @implements {Activatable}
  */
 export class Nearest {
+  /**
+   * @param {InteractorMark} mark The mark to interact with.
+   * @param {*} options The interactor options.
+   */
   constructor(mark, {
     selection,
     pointer,
@@ -49,7 +54,7 @@ export class Nearest {
     // eslint-disable-next-line @typescript-eslint/no-this-alias
     const that = this;
     const { mark, channels, selection, maxRadius } = this;
-    const { data: { columns } } = mark;
+    const columns = mark.data && 'columns' in mark.data ? mark.data.columns : {};
     const keys = channels.map(c => mark.channelField(c).as);
     const param = !isSelection(selection);
 
diff --git a/packages/vgplot/plot/src/interactors/Toggle.js b/packages/vgplot/plot/src/interactors/Toggle.js
index 911654a7..36afcaea 100644
--- a/packages/vgplot/plot/src/interactors/Toggle.js
+++ b/packages/vgplot/plot/src/interactors/Toggle.js
@@ -1,4 +1,5 @@
 /** @import { ClauseSource } from '@uwdata/mosaic-core' */
+/** @import { InteractorMark } from '../marks/Mark.js' */
 import { clauseList, clausePoints } from '@uwdata/mosaic-core';
 import { getDatum } from './util/get-datum.js';
 import { neq, neqSome } from './util/neq.js';
@@ -9,7 +10,7 @@ import { neq, neqSome } from './util/neq.js';
  */
 export class Toggle {
   /**
-   * @param {*} mark The mark to interact with.
+   * @param {InteractorMark} mark The mark to interact with.
    * @param {*} options The interactor options.
    */
   constructor(mark, {
@@ -58,7 +59,7 @@ export class Toggle {
 
   init(svg, selector, accessor) {
     const { mark, as, selection } = this;
-    const { data: { columns = {} } = {} } = mark;
+    const columns = mark.data && 'columns' in mark.data ? mark.data.columns : {};
     accessor ??= target => as.map(name => columns[name][getDatum(target)]);
 
     selector ??= `[data-index="${mark.index}"]`;
diff --git a/packages/vgplot/plot/src/legend.js b/packages/vgplot/plot/src/legend.js
index f01763ff..a9cd9565 100644
--- a/packages/vgplot/plot/src/legend.js
+++ b/packages/vgplot/plot/src/legend.js
@@ -132,12 +132,20 @@ function getInteractor(legend, type) {
 }
 
 // generate a faux mark to pass to an interactor
+/**
+ * @import { InteractorMark } from './marks/Mark.js'
+ * @returns {InteractorMark | undefined}
+ */
 function interactorMark(legend) {
   const { channel, plot } = legend;
   const field = legend.field ?? findField(plot.marks, channel) ?? 'value';
   if (field) {
     const f = { field };
-    return { plot, channelField: c => channel === c ? f : undefined };
+    return {
+      plot,
+      channelField: c => channel === c ? f : undefined,
+      isUnnested: () => false
+    };
   }
 }
 
diff --git a/packages/vgplot/plot/src/marks/Mark.js b/packages/vgplot/plot/src/marks/Mark.js
index 25212f58..8e779604 100644
--- a/packages/vgplot/plot/src/marks/Mark.js
+++ b/packages/vgplot/plot/src/marks/Mark.js
@@ -1,4 +1,5 @@
 /** @import { SelectQuery } from '@uwdata/mosaic-sql' */
+/** @import { DataColumns } from '@uwdata/mosaic-core' */
 import { isParam, MosaicClient, queryFieldInfo, toDataColumns } from '@uwdata/mosaic-core';
 import { Query, collectParams, column, isAggregateExpression, isColumnParam, isColumnRef, isNode, isParamLike, unnest } from '@uwdata/mosaic-sql';
 import { isColor } from './util/is-color.js';
@@ -30,6 +31,26 @@ const valueEntry = (channel, value) => ({ channel, value });
 // as opposed to a database table reference
 export const isDataArray = source => Array.isArray(source);
 
+/**
+ * The minimal mark surface consumed by interactors, allowing marks
+ * and lightweight stand-ins to be used interchangeably.
+ * @typedef {object} InteractorMark
+ * @property {import('../plot.js').Plot} plot
+ *   The plot the mark belongs to.
+ * @property {(channel: string, options?: { exact?: boolean }) =>
+ *   ({ field?: any, as?: any } | null | undefined)} channelField
+ *   Look up the channel entry bound to a channel, if any.
+ * @property {(field: any) => boolean} isUnnested
+ *   Whether the given field is unnested by the mark's source.
+ * @property {DataColumns} [data]
+ *   Materialized column data, when available.
+ * @property {number} [index]
+ *   The mark's index within the plot.
+ */
+
+/**
+ * @implements {InteractorMark}
+ */
 export class Mark extends MosaicClient {
   constructor(type, source, encodings, reqs = {}) {
     super(source?.options?.filterBy);