aspectRatio option

README.md

@@ -92,9 +92,12 @@ These options determine the overall layout of the plot; all are specified as num
 * **margin** - shorthand for the four margins
 * **width** - the outer width of the plot (including margins)
 * **height** - the outer height of the plot (including margins)
+* **aspectRatio** - the desired aspect ratio of data (affecting default **height**)
 
 The default **width** is 640. On Observable, the width can be set to the [standard width](https://github.com/observablehq/stdlib/blob/main/README.md#width) to make responsive plots. The default **height** is chosen automatically based on the plot’s associated scales; for example, if *y* is linear and there is no *fy* scale, it might be 396. The default margins depend on the maximum margins of the plot’s constituent [marks](#mark-options). While most marks default to zero margins (because they are drawn inside the chart area), Plot’s [axis mark](#axis) has non-zero default margins.
 
+The **aspectRatio** option, if not null, computes a default **height** such that a variation of one unit in the *x* dimension is represented by the corresponding number of pixels as a variation in the *y* dimension of one unit. Note: when using facets, set the *fx* and *fy* scales’ **round** option to false if you need an exact aspect ratio.
+
 The **style** option allows custom styles to override Plot’s defaults. It may be specified either as a string of inline styles (*e.g.*, `"color: red;"`, in the same fashion as assigning [*element*.style](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/style)) or an object of properties (*e.g.*, `{color: "red"}`, in the same fashion as assigning [*element*.style properties](https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleDeclaration)). Note that unitless numbers ([quirky lengths](https://www.w3.org/TR/css-values-4/#deprecated-quirky-length)) such as `{padding: 20}` may not supported by some browsers; you should instead specify a string with units such as `{padding: "20px"}`. By default, the returned plot has a white background, a max-width of 100%, and the system-ui font. Plot’s marks and axes default to [currentColor](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#currentcolor_keyword), meaning that they will inherit the surrounding content’s color. For example, a dark theme:
 
 ```js

src/dimensions.js

@@ -1,3 +1,4 @@
+import {extent} from "d3";
 import {projectionAspectRatio} from "./projection.js";
 import {isOrdinalScale} from "./scales.js";
 import {offset} from "./style.js";
@@ -87,9 +88,9 @@ export function Dimensions(scales, marks, options = {}) {
 }
 
 function autoHeight(
-  {y, fy, fx},
+  {x, y, fy, fx},
   marks,
-  {projection},
+  {projection, aspectRatio},
   {width, marginTopDefault, marginRightDefault, marginBottomDefault, marginLeftDefault}
 ) {
   const nfy = fy ? fy.scale.domain().length : 1;
@@ -104,5 +105,45 @@ function autoHeight(
   }
 
   const ny = y ? (isOrdinalScale(y) ? y.scale.domain().length : Math.max(7, 17 / nfy)) : 1;
+
+  // If a desired aspect ratio is given, compute a default height to match.
+  if (aspectRatio != null) {
+    aspectRatio = +aspectRatio;
+    if (!(isFinite(aspectRatio) && aspectRatio > 0)) throw new Error(`invalid aspectRatio: ${aspectRatio}`);
+    const ratio = aspectRatioLength("y", y) / (aspectRatioLength("x", x) * aspectRatio);
+    const fxb = fx ? fx.scale.bandwidth() : 1;
+    const fyb = fy ? fy.scale.bandwidth() : 1;
+    const w = fxb * (width - marginLeftDefault - marginRightDefault) - x.insetLeft - x.insetRight;
+    return (ratio * w + y.insetTop + y.insetBottom) / fyb + marginTopDefault + marginBottomDefault;
+  }
+
   return !!(y || fy) * Math.max(1, Math.min(60, ny * nfy)) * 20 + !!fx * 30 + 60;
 }
+
+function aspectRatioLength(k, scale) {
+  if (!scale) throw new Error(`aspectRatio requires ${k} scale`);
+  const {type, domain} = scale;
+  let transform;
+  switch (type) {
+    case "linear":
+    case "utc":
+    case "time":
+      transform = Number;
+      break;
+    case "pow": {
+      const exponent = scale.scale.exponent();
+      transform = (x) => Math.pow(x, exponent);
+      break;
+    }
+    case "log":
+      transform = Math.log;
+      break;
+    case "point":
+    case "band":
+      return domain.length;
+    default:
+      throw new Error(`unsupported ${k} scale for aspectRatio: ${type}`);
+  }
+  const [min, max] = extent(domain);
+  return Math.abs(transform(max) - transform(min));
+}

test/data/README.md

@@ -95,6 +95,11 @@ https://data.giss.nasa.gov/gistemp/
 Met Office Hadley Centre
 https://www.metoffice.gov.uk/hadobs/hadcrut4/data/current/series_format.html
 
+## libor.csv
+CBO
+https://www.cbo.gov/topics/budget/accuracy-projections
+https://observablehq.com/@tophtucker/examples-of-bitemporal-charts
+
 ## metros.csv
 The New York Times
 https://www.nytimes.com/2019/12/02/upshot/wealth-poverty-divide-american-cities.html