extend interval to dot and text marks

README.md

@@ -1045,6 +1045,8 @@ Plot.dotX(cars.map(d => d["economy (mpg)"]))
 
 Equivalent to [Plot.dot](#plotdotdata-options) except that if the **x** option is not specified, it defaults to the identity function and assumes that *data* = [*x₀*, *x₁*, *x₂*, …].
 
+If an **interval** is specified, such as d3.utcDay, **y** is transformed to (*interval*.floor(*y*) + *interval*.offset(*interval*.floor(*y*))) / 2. If the interval is specified as a number *n*, *y* will be the midpoint of two consecutive multiples of *n* that bracket *y*.
+
 #### Plot.dotY(*data*, *options*)
 
 ```js
@@ -1053,6 +1055,8 @@ Plot.dotY(cars.map(d => d["economy (mpg)"]))
 
 Equivalent to [Plot.dot](#plotdotdata-options) except that if the **y** option is not specified, it defaults to the identity function and assumes that *data* = [*y₀*, *y₁*, *y₂*, …].
 
+If an **interval** is specified, such as d3.utcDay, **x** is transformed to (*interval*.floor(*x*) + *interval*.offset(*interval*.floor(*x*))) / 2. If the interval is specified as a number *n*, *x* will be the midpoint of two consecutive multiples of *n* that bracket *x*.
+
 ### Hexgrid
 
 The hexgrid mark can be used to support marks using the [hexbin](#hexbin) layout.
@@ -1327,10 +1331,14 @@ Returns a new text mark with the given *data* and *options*. If neither the **x*
 
 Equivalent to [Plot.text](#plottextdata-options), except **x** defaults to the identity function and assumes that *data* = [*x₀*, *x₁*, *x₂*, …].
 
+If an **interval** is specified, such as d3.utcDay, **y** is transformed to (*interval*.floor(*y*) + *interval*.offset(*interval*.floor(*y*))) / 2. If the interval is specified as a number *n*, *y* will be the midpoint of two consecutive multiples of *n* that bracket *y*.
+
 #### Plot.textY(*data*, *options*)
 
 Equivalent to [Plot.text](#plottextdata-options), except **y** defaults to the identity function and assumes that *data* = [*y₀*, *y₁*, *y₂*, …].
 
+If an **interval** is specified, such as d3.utcDay, **x** is transformed to (*interval*.floor(*x*) + *interval*.offset(*interval*.floor(*x*))) / 2. If the interval is specified as a number *n*, *x* will be the midpoint of two consecutive multiples of *n* that bracket *x*.
+
 ### Tick
 
 [<img src="./img/tick.png" width="320" height="198" alt="a barcode plot">](https://observablehq.com/@observablehq/plot-tick)

src/marks/dot.js

@@ -4,6 +4,7 @@ import {identity, maybeFrameAnchor, maybeNumberChannel, maybeTuple} from "../opt
 import {Mark} from "../plot.js";
 import {applyChannelStyles, applyDirectStyles, applyFrameAnchor, applyIndirectStyles, applyTransform, offset} from "../style.js";
 import {maybeSymbolChannel} from "../symbols.js";
+import {maybeIntervalMidX, maybeIntervalMidY} from "../transforms/interval.js";
 
 const defaults = {
   ariaLabel: "dot",
@@ -95,11 +96,11 @@ export function dot(data, {x, y, ...options} = {}) {
 }
 
 export function dotX(data, {x = identity, ...options} = {}) {
-  return new Dot(data, {...options, x});
+  return new Dot(data, maybeIntervalMidY({...options, x}));
 }
 
 export function dotY(data, {y = identity, ...options} = {}) {
-  return new Dot(data, {...options, y});
+  return new Dot(data, maybeIntervalMidX({...options, y}));
 }
 
 export function circle(data, options) {

src/marks/text.js

@@ -4,6 +4,7 @@ import {formatDefault} from "../format.js";
 import {indexOf, identity, string, maybeNumberChannel, maybeTuple, numberChannel, isNumeric, isTemporal, keyword, maybeFrameAnchor, isTextual} from "../options.js";
 import {Mark} from "../plot.js";
 import {applyChannelStyles, applyDirectStyles, applyIndirectStyles, applyAttr, applyTransform, offset, impliedString, applyFrameAnchor} from "../style.js";
+import {maybeIntervalMidX, maybeIntervalMidY} from "../transforms/interval.js";
 
 const defaults = {
   ariaLabel: "text",
@@ -122,11 +123,11 @@ export function text(data, {x, y, ...options} = {}) {
 }
 
 export function textX(data, {x = identity, ...options} = {}) {
-  return new Text(data, {...options, x});
+  return new Text(data, maybeIntervalMidY({...options, x}));
 }
 
 export function textY(data, {y = identity, ...options} = {}) {
-  return new Text(data, {...options, y});
+  return new Text(data, maybeIntervalMidX({...options, y}));
 }
 
 function applyIndirectTextStyles(selection, mark, T) {

src/transforms/interval.js

@@ -1,5 +1,5 @@
 import {range} from "d3";
-import {labelof, maybeValue, valueof} from "../options.js";
+import {isTemporal, labelof, maybeValue, valueof} from "../options.js";
 import {maybeInsetX, maybeInsetY} from "./inset.js";
 
 // TODO Allow the interval to be specified as a string, e.g. “day” or “hour”?
@@ -43,13 +43,35 @@ function maybeIntervalK(k, maybeInsetK, options, trivial) {
       [`${k}2`]: v2 === undefined ? kv : v2
     };
   }
-  let V1;
-  const tv1 = data => V1 || (V1 = valueof(data, value).map(v => interval.floor(v)));
+  let D1, V1;
+  function transform(data) {
+    if (V1 !== undefined && data === D1) return V1; // memoize
+    return V1 = Array.from(valueof(D1 = data, value), v => interval.floor(v));
+  }
   return maybeInsetK({
     ...options,
     [k]: undefined,
-    [`${k}1`]: v1 === undefined ? {transform: tv1, label} : v1,
-    [`${k}2`]: v2 === undefined ? {transform: data => tv1(data).map(v => interval.offset(v)), label} : v2
+    [`${k}1`]: v1 === undefined ? {transform, label} : v1,
+    [`${k}2`]: v2 === undefined ? {transform: data => transform(data).map(v => interval.offset(v)), label} : v2
+  });
+}
+
+function maybeIntervalMidK(k, maybeInsetK, options) {
+  const {[k]: v} = options;
+  const {value, interval} = maybeIntervalValue(v, options);
+  if (value == null || interval == null) return options;
+  return maybeInsetK({
+    ...options,
+    [k]: {
+      label: labelof(v),
+      transform: data => {
+        const V1 = Array.from(valueof(data, value), v => interval.floor(v));
+        const V2 = V1.map(v => interval.offset(v));
+        return V1.map(isTemporal(V1)
+          ? (v1, v2) => v1 == null || isNaN(v1 = +v1) || (v2 = V2[v2], v2 == null) || isNaN(v2 = +v2) ? undefined : new Date((v1 + v2) / 2)
+          : (v1, v2) => v1 == null || (v2 = V2[v2], v2 == null) ? NaN : (+v1 + +v2) / 2);
+      }
+    }
   });
 }
 
@@ -68,3 +90,11 @@ export function maybeIntervalX(options = {}) {
 export function maybeIntervalY(options = {}) {
   return maybeIntervalK("y", maybeInsetY, options);
 }
+
+export function maybeIntervalMidX(options = {}) {
+  return maybeIntervalMidK("x", maybeInsetX, options);
+}
+
+export function maybeIntervalMidY(options = {}) {
+  return maybeIntervalMidK("y", maybeInsetY, options);
+}