tip title format

docs/marks/tip.md

@@ -131,7 +131,7 @@ Plot.rectY(olympians, Plot.binX({y: "sum"}, {x: "weight", y: (d) => d.sex === "m
 ```
 :::
 
-The order and formatting of channels in the tip can be customized with the **format** option <VersionBadge version="0.6.11" pr="1823" />, which accepts a key-value object mapping channel names to formats. Each [format](../features/formats.md) can be a string (for number or time formats), a function that receives the value as input and returns a string, true to use the default format, and null or false to suppress. The order of channels in the tip follows their order in the format object followed by any additional channels.
+The order and formatting of channels in the tip can be customized with the **format** option <VersionBadge version="0.6.11" pr="1823" />, which accepts a key-value object mapping channel names to formats. Each [format](../features/formats.md) can be a string (for number or time formats), a function that receives the value as input and returns a string, true to use the default format, and null or false to suppress. The order of channels in the tip follows their order in the format object followed by any additional channels. When using the **title** channel, the **format** option may be specified as a string or a function; the given format will then apply to the **title** channel. <VersionBadge pr="2074" />
 
 A channel’s label can be specified alongside its value as a {value, label} object; if a channel label is not specified, the associated scale’s label is used, if any; if there is no associated scale, or if the scale has no label, the channel name is used instead.
 

src/marks/tip.d.ts

@@ -74,7 +74,7 @@ export interface TipOptions extends MarkOptions, TextStyles {
    * is interpreted as a (UTC) time format for temporal channels, and otherwise
    * a number format.
    */
-  format?: {[name in ChannelName]?: boolean | string | ((d: any, i: number) => string)};
+  format?: {[name in ChannelName]?: null | boolean | TipFormat} | TipFormat;
 
   /** The image filter for the tip’s box; defaults to a drop shadow. */
   pathFilter?: string;
@@ -86,6 +86,18 @@ export interface TipOptions extends MarkOptions, TextStyles {
   textPadding?: number;
 }
 
+/**
+ * How to format channel values; one of:
+ *
+ * - a [d3-format][1] string for numeric scales
+ * - a [d3-time-format][2] string for temporal scales
+ * - a function passed a channel *value* and *index*, returning a string
+ *
+ * [1]: https://d3js.org/d3-time
+ * [2]: https://d3js.org/d3-time-format
+ */
+export type TipFormat = string | ((d: any, i: number) => string);
+
 /**
  * Returns a new tip mark for the given *data* and *options*.
  *

src/marks/tip.js

@@ -84,7 +84,7 @@ export class Tip extends Mark {
     for (const key in defaults) if (key in this.channels) this[key] = defaults[key]; // apply default even if channel
     this.splitLines = splitter(this);
     this.clipLine = clipper(this);
-    this.format = {...format}; // defensive copy before mutate; also promote nullish to empty
+    this.format = typeof format === "string" || typeof format === "function" ? {title: format} : {...format}; // defensive copy before mutate; also promote nullish to empty
   }
   render(index, scales, values, dimensions, context) {
     const mark = this;
@@ -120,10 +120,10 @@ export class Tip extends Mark {
     // channels as name-value pairs.
     let sources, format;
     if ("title" in values) {
-      sources = values.channels;
+      sources = getSourceChannels.call(this, {title: values.channels.title}, scales);
       format = formatTitle;
     } else {
-      sources = getSourceChannels.call(this, values, scales);
+      sources = getSourceChannels.call(this, values.channels, scales);
       format = formatChannels;
     }
 
@@ -319,7 +319,7 @@ function getPath(anchor, m, r, width, height) {
 }
 
 // Note: mutates this.format!
-function getSourceChannels({channels}, scales) {
+function getSourceChannels(channels, scales) {
   const sources = {};
 
   // Promote x and y shorthand for paired channels (in order).
@@ -384,7 +384,7 @@ function maybeExpandPairedFormat(format, channels, key) {
 }
 
 function formatTitle(i, index, {title}) {
-  return formatDefault(title.value[i], i);
+  return this.format.title(title.value[i], i);
 }
 
 function* formatChannels(i, index, channels, scales, values) {

src/scales.d.ts

@@ -588,7 +588,7 @@ export interface ScaleOptions extends ScaleDefaults {
    * [1]: https://d3js.org/d3-time
    * [2]: https://d3js.org/d3-time-format
    */
-  tickFormat?: string | ((t: any, i: number) => any) | null;
+  tickFormat?: string | ((d: any, i: number) => any) | null;
 
   /**
    * The rotation angle of axis tick labels in degrees clocksize; defaults to 0.

test/plots/tip-format.ts

@@ -104,8 +104,16 @@ export async function tipFormatTitleExplicit() {
   return tip({length: 1}, {title: [new Date("2010-01-01")]});
 }
 
-export async function tipFormatTitleIgnoreFormat() {
-  return tip({length: 1}, {title: [0], format: {title: ".2f"}});
+export async function tipFormatTitleFormat() {
+  return tip({length: 1}, {title: [0.009], format: {title: ".2f"}});
+}
+
+export async function tipFormatTitleFormatFunction() {
+  return tip({length: 1}, {title: [0.019], format: (d) => d.toFixed(2)});
+}
+
+export async function tipFormatTitleFormatShorthand() {
+  return tip({length: 1}, {title: [0.029], format: ".2f"});
 }
 
 export async function tipFormatTitlePrimitive() {