dataset mark option

docs/features/marks.md

@@ -510,6 +510,7 @@ All marks support the following optional channels:
 * **title** - an accessible, short-text description (a string of text, possibly with newlines)
 * **href** - a URL to link to
 * **ariaLabel** - a short label representing the value in the accessibility tree
+* **dataset** - the [dataset property](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/dataset) <VersionBadge pr="2295" />
 
 The **fill**, **fillOpacity**, **stroke**, **strokeWidth**, **strokeOpacity**, and **opacity** options can be specified as either channels or constants. When the fill or stroke is specified as a function or array, it is interpreted as a channel; when the fill or stroke is specified as a string, it is interpreted as a constant if a valid CSS color and otherwise it is interpreted as a column name for a channel. Similarly when the fill opacity, stroke opacity, object opacity, stroke width, or radius is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel.
 
@@ -533,7 +534,9 @@ In addition to functions of data, arrays, and column names, channel values can b
 Plot.dot(numbers, {x: {transform: (data) => data}})
 ```
 
-The **title**, **href**, and **ariaLabel** options can *only* be specified as channels. When these options are specified as a string, the string refers to the name of a column in the mark’s associated data. If you’d like every instance of a particular mark to have the same value, specify the option as a function that returns the desired value, *e.g.* `() => "Hello, world!"`.
+The **title**, **href**, **ariaLabel**, and **dataset** options can *only* be specified as channels. When these options are specified as a string, the string refers to the name of a column in the mark’s associated data. If you’d like every instance of a particular mark to have the same value, specify the option as a function that returns the desired value, *e.g.* `() => "Hello, world!"`.
+
+When the **dataset** channel contains boolean, number, string or date values, they are applied as a `data-{key}` property, where the key is the channel’s label if present, and otherwise defaults to "value". When the values are objects, each entry is applied individually as a `data-{key}` property. Values are coerced to strings, with dates in [short ISO format](https://github.com/mbostock/isoformat). Keys must follow the naming specification for [custom data attributes](https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes).
 
 For marks that support the **frameAnchor** option, it may be specified as one of the four sides (*top*, *right*, *bottom*, *left*), one of the four corners (*top-left*, *top-right*, *bottom-right*, *bottom-left*), or the *middle* of the frame.
 

src/mark.d.ts

@@ -466,6 +466,21 @@ export interface MarkOptions {
    */
   target?: string;
 
+  /**
+   * A [dataset][1] for the mark; a channel specifying arbitrary data. When the
+   * **dataset** channel contains boolean, number, string or date values, they
+   * are applied as a `data-{key}` property, where the key is the channel’s
+   * label if present, and otherwise defaults to "value". When the values are
+   * objects, each entry is applied individually as a `data-{key}` property.
+   * Values are coerced to strings, with dates in [short ISO format][2]. Keys
+   * must follow the naming specification for [custom data attributes][3].
+   *
+   * [1]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/dataset
+   * [2]: https://github.com/mbostock/isoformat
+   * [3]: https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes
+   */
+  dataset?: ChannelValueSpec;
+
   /**
    * An object defining additional custom channels. This meta option may be used
    * by an **initializer** to declare extra channels.

src/style.js

@@ -1,7 +1,7 @@
 import {group, namespaces, select} from "d3";
 import {create} from "./context.js";
 import {defined, nonempty} from "./defined.js";
-import {formatDefault} from "./format.js";
+import {formatDefault, formatIsoDate} from "./format.js";
 import {isNone, isNoneish, isRound, maybeColorChannel, maybeNumberChannel} from "./options.js";
 import {keyof, number, string} from "./options.js";
 import {warn} from "./warnings.js";
@@ -44,7 +44,8 @@ export function styles(
     paintOrder,
     pointerEvents,
     shapeRendering,
-    channels
+    channels,
+    dataset
   },
   {
     ariaLabel: cariaLabel,
@@ -137,6 +138,7 @@ export function styles(
   mark.paintOrder = impliedString(paintOrder, "normal");
   mark.pointerEvents = impliedString(pointerEvents, "auto");
   mark.shapeRendering = impliedString(shapeRendering, "auto");
+  mark.datakey = typeof dataset === "string" ? dataset : dataset?.label;
 
   return {
     title: {value: title, optional: true, filter: null},
@@ -147,7 +149,8 @@ export function styles(
     stroke: {value: vstroke, scale: "auto", optional: true},
     strokeOpacity: {value: vstrokeOpacity, scale: "auto", optional: true},
     strokeWidth: {value: vstrokeWidth, optional: true},
-    opacity: {value: vopacity, scale: "auto", optional: true}
+    opacity: {value: vopacity, scale: "auto", optional: true},
+    dataset: {value: dataset, optional: true, filter: null}
   };
 }
 
@@ -179,7 +182,7 @@ export function applyTextGroup(selection, T) {
 
 export function applyChannelStyles(
   selection,
-  {target, tip},
+  {target, tip, datakey},
   {
     ariaLabel: AL,
     title: T,
@@ -189,7 +192,8 @@ export function applyChannelStyles(
     strokeOpacity: SO,
     strokeWidth: SW,
     opacity: O,
-    href: H
+    href: H,
+    dataset: D
   }
 ) {
   if (AL) applyAttr(selection, "aria-label", (i) => AL[i]);
@@ -200,6 +204,7 @@ export function applyChannelStyles(
   if (SW) applyAttr(selection, "stroke-width", (i) => SW[i]);
   if (O) applyAttr(selection, "opacity", (i) => O[i]);
   if (H) applyHref(selection, (i) => H[i], target);
+  if (D) applyDataset(selection, (i) => D[i], datakey);
   if (!tip) applyTitle(selection, T);
 }
 
@@ -422,6 +427,21 @@ export function applyTransform(selection, mark, {x, y}, tx = offset, ty = offset
   if (tx || ty) selection.attr("transform", `translate(${tx},${ty})`);
 }
 
+export function applyDataset(selection, value, keyname = "value") {
+  selection.each(function (i) {
+    const V = value(i);
+    if (V == null) return;
+    const O =
+      typeof V === "number" || typeof V === "boolean" || typeof V === "string" || V instanceof Date
+        ? {[keyname]: V}
+        : V;
+    if (typeof O !== "object") throw new Error(`Unsupported dataset property: ${value}`);
+    for (const [key, v] of Object.entries(O)) {
+      this.setAttribute(`data-${key}`, v instanceof Date ? formatIsoDate(v) : String(v));
+    }
+  });
+}
+
 export function impliedString(value, impliedValue) {
   if ((value = string(value)) !== impliedValue) return value;
 }