rect support for band scales

docs/marks/bar.md

@@ -26,7 +26,7 @@ const timeseries = [
 # Bar mark
 
 :::tip
-The bar mark is one of several marks in Plot for drawing rectangles; it should be used when one dimension is ordinal and the other is quantitative. See also [rect](./rect.md) and [cell](./cell.md).
+The bar mark is a variant of the [rect mark](./rect.md) for use when one dimension is ordinal and the other is quantitative. See also the [cell mark](./cell.md).
 :::
 
 The **bar mark** comes in two orientations: [barY](#barY) extends vertically↑ as in a vertical bar chart or column chart, while [barX](#barX) extends horizontally→. For example, the bar chart below shows the frequency of letters in the English language.

docs/marks/cell.md

@@ -21,7 +21,7 @@ onMounted(() => {
 # Cell mark
 
 :::tip
-The cell mark is one of several marks in Plot for drawing rectangles; it should be used when both dimensions are ordinal. See also [bar](./bar.md) and [rect](./rect.md).
+The cell mark is a variant of the [rect mark](./rect.md) for use when both dimensions are ordinal. See also the [bar mark](./bar.md).
 :::
 
 The **cell mark** draws rectangles positioned in two ordinal dimensions. Hence, the plot’s *x* and *y* scales are [band scales](../features/scales.md). Cells typically also have a **fill** color encoding.

docs/marks/rect.md

@@ -26,10 +26,6 @@ onMounted(() => {
 
 # Rect mark
 
-:::tip
-The rect mark is one of several marks in Plot for drawing rectangles; it should be used when both dimensions are quantitative. See also [bar](./bar.md) and [cell](./cell.md).
-:::
-
 The **rect mark** draws axis-aligned rectangles defined by **x1**, **y1**, **x2**, and **y2**. For example, here we display geographic bounding boxes of U.S. counties represented as [*x1*, *y1*, *x2*, *y2*] tuples, where *x1* & *x2* are degrees longitude and *y1* & *y2* are degrees latitude.
 
 :::plot defer https://observablehq.com/@observablehq/plot-county-boxes
@@ -199,7 +195,7 @@ The following channels are optional:
 * **x2** - the ending horizontal position; bound to the *x* scale
 * **y2** - the ending vertical position; bound to the *y* scale
 
-Typically either **x1** and **x2** are specified, or **y1** and **y2**, or both.
+If **x1** is specified but **x2** is not specified, then *x* must be a *band* scale; if **y1** is specified but **y2** is not specified, then *y* must be a *band* scale.
 
 If an **interval** is specified, such as d3.utcDay, **x1** and **x2** can be derived from **x**: *interval*.floor(*x*) is invoked for each **x** to produce **x1**, and *interval*.offset(*x1*) is invoked for each **x1** to produce **x2**. The same is true for **y**, **y1**, and **y2**, respectively. If the interval is specified as a number *n*, **x1** and **x2** are taken as the two consecutive multiples of *n* that bracket **x**. Named UTC intervals such as *day* are also supported; see [scale options](../features/scales.md#scale-options).
 

src/marks/rect.js

@@ -30,8 +30,8 @@ export class Rect extends Mark {
     super(
       data,
       {
-        x1: {value: x1, scale: "x", optional: true},
-        y1: {value: y1, scale: "y", optional: true},
+        x1: {value: x1, scale: "x", type: x1 != null && x2 == null ? "band" : undefined, optional: true},
+        y1: {value: y1, scale: "y", type: y1 != null && y2 == null ? "band" : undefined, optional: true},
         x2: {value: x2, scale: "x", optional: true},
         y2: {value: y2, scale: "y", optional: true}
       },
@@ -51,9 +51,11 @@ export class Rect extends Mark {
     const {marginTop, marginRight, marginBottom, marginLeft, width, height} = dimensions;
     const {projection} = context;
     const {insetTop, insetRight, insetBottom, insetLeft, rx, ry} = this;
+    const bx = (x?.bandwidth ? x.bandwidth() : 0) - insetLeft - insetRight;
+    const by = (y?.bandwidth ? y.bandwidth() : 0) - insetTop - insetBottom;
     return create("svg:g", context)
       .call(applyIndirectStyles, this, dimensions, context)
-      .call(applyTransform, this, {x: X1 && X2 && x, y: Y1 && Y2 && y}, 0, 0)
+      .call(applyTransform, this, {}, 0, 0)
       .call((g) =>
         g
           .selectAll()
@@ -63,26 +65,34 @@ export class Rect extends Mark {
           .call(applyDirectStyles, this)
           .attr(
             "x",
-            X1 && X2 && (projection || !isCollapsed(x))
-              ? (i) => Math.min(X1[i], X2[i]) + insetLeft
+            X1 && (projection || !isCollapsed(x))
+              ? X2
+                ? (i) => Math.min(X1[i], X2[i]) + insetLeft
+                : (i) => X1[i] + insetLeft
               : marginLeft + insetLeft
           )
           .attr(
             "y",
-            Y1 && Y2 && (projection || !isCollapsed(y))
-              ? (i) => Math.min(Y1[i], Y2[i]) + insetTop
+            Y1 && (projection || !isCollapsed(y))
+              ? Y2
+                ? (i) => Math.min(Y1[i], Y2[i]) + insetTop
+                : (i) => Y1[i] + insetTop
               : marginTop + insetTop
           )
           .attr(
             "width",
-            X1 && X2 && (projection || !isCollapsed(x))
-              ? (i) => Math.max(0, Math.abs(X2[i] - X1[i]) - insetLeft - insetRight)
+            X1 && (projection || !isCollapsed(x))
+              ? X2
+                ? (i) => Math.max(0, Math.abs(X2[i] - X1[i]) + bx)
+                : bx
               : width - marginRight - marginLeft - insetRight - insetLeft
           )
           .attr(
             "height",
-            Y1 && Y2 && (projection || !isCollapsed(y))
-              ? (i) => Math.max(0, Math.abs(Y1[i] - Y2[i]) - insetTop - insetBottom)
+            Y1 && (projection || !isCollapsed(y))
+              ? Y2
+                ? (i) => Math.max(0, Math.abs(Y1[i] - Y2[i]) + by)
+                : by
               : height - marginTop - marginBottom - insetTop - insetBottom
           )
           .call(applyAttr, "rx", rx)

src/transforms/interval.js

@@ -22,7 +22,7 @@ function maybeIntervalK(k, maybeInsetK, options, trivial) {
       ...options,
       [k]: undefined,
       [`${k}1`]: v1 === undefined ? kv : v1,
-      [`${k}2`]: v2 === undefined ? kv : v2
+      [`${k}2`]: v2 === undefined && !(v1 === v2 && trivial) ? kv : v2
     };
   }
   let D1, V1;