fix: handle stepped scales for autosized facets

src/compile/layoutsize/assemble.ts

@@ -5,7 +5,9 @@ import {getFirstDefined} from '../../util';
 import {isVgRangeStep, VgRangeStep} from '../../vega.schema';
 import {isFacetModel, Model} from '../model';
 import {ScaleComponent} from '../scale/component';
-import {LayoutSizeType} from './component';
+import {getSizeTypeFromLayoutSizeType, LayoutSizeType} from './component';
+import {isFacetMapping} from '../../spec/facet';
+import {FacetModel} from "../facet";
 
 export function assembleLayoutSignals(model: Model): NewSignal[] {
   return [
@@ -26,7 +28,14 @@ export function sizeSignals(model: Model, sizeType: LayoutSizeType): (NewSignal
   // Read size signal name from name map, just in case it is the top-level size signal that got renamed.
   const name = model.getSizeSignalRef(sizeType).signal;
 
-  if (size === 'step') {
+  if (isFacetModel(model.parent) && model.parent.hasStaticOuterDimension(getSizeTypeFromLayoutSizeType(sizeType))) {
+    return [
+      {
+        name,
+        update: autosizedFacetExpr(model.parent, sizeType)
+      }
+    ];
+  } else if (size === 'step') {
     const scaleComponent = model.getScaleComponent(channel);
 
     if (scaleComponent) {
@@ -63,34 +72,6 @@ export function sizeSignals(model: Model, sizeType: LayoutSizeType): (NewSignal
     const defaultValue = getViewConfigContinuousSize(model.config.view, isWidth ? 'width' : 'height');
     const safeExpr = `isFinite(${expr}) ? ${expr} : ${defaultValue}`;
     return [{name, init: safeExpr, on: [{update: safeExpr, events: 'window:resize'}]}];
-  } else if (isFacetModel(model.parent) && sizeType === 'width' && model.parent.size.width) {
-    return [
-      {
-        name,
-        // if the facet operator defines a column channel, the compiled vega spec includes the 'column_domain' data
-        // if however the facet operator is itself is a facet field definition, the compiled vega spec instead includes the 'facet_domain_column' data
-        // otherwise there is no column faceting so the width should be passed through
-        update: model.parent.facet.column
-          ? "width / length(data('column_domain'))"
-          : model.parent.facet.facet
-          ? "width / length(data('facet_domain_column'))"
-          : 'width'
-      }
-    ];
-  } else if (isFacetModel(model.parent) && sizeType === 'height' && model.parent.size.height) {
-    return [
-      {
-        name,
-        // if the facet operator defines a row channel, the compiled vega spec includes the 'row_domain' data
-        // if however the facet operator is itself is a facet field definition, the compiled vega spec instead includes the 'facet_domain_row' data
-        // otherwise there is no row faceting so the height should be passed through
-        update: model.parent.facet.row
-          ? "height / length(data('row_domain'))"
-          : model.parent.facet.facet
-          ? "height / length(data('facet_domain_row'))"
-          : 'height'
-      }
-    ];
   } else {
     return [
       {
@@ -125,3 +106,15 @@ export function sizeExpr(scaleName: string, scaleComponent: ScaleComponent, card
         1;
   return `bandspace(${cardinality}, ${paddingInner}, ${paddingOuter}) * ${scaleName}_step`;
 }
+
+export function autosizedFacetExpr(model: FacetModel, sizeType: LayoutSizeType) {
+  const channel = sizeType === 'width' ? 'column' : 'row';
+  if (!model.facet[channel] && isFacetMapping(model.facet)) {
+    // no faceting on this channel so just pass through the overall dimension of this channel
+    return sizeType;
+  }
+  // if the facet operator defines an explicit row or column channel, the compiled vega spec includes the '(row|column)_domain' data
+  // otherwise the facet operator is itself is a facet field definition, the compiled vega spec instead includes the 'facet_domain_(row|column)' data
+  const domain = !isFacetMapping(model.facet) ? `facet_domain_${channel}` : `${channel}_domain`;
+  return `${sizeType} / length(data('${domain}'))`;
+}

src/compile/model.ts

@@ -28,6 +28,8 @@ import {isFacetSpec} from '../spec';
 import {
   extractCompositionLayout,
   GenericCompositionLayoutWithColumns,
+  isStep,
+  LayoutSizeField,
   LayoutSizeMixins,
   SpecType,
   ViewBackground
@@ -658,6 +660,13 @@ export abstract class Model {
       this.component.axes.y?.some(a => a.hasOrientSignalRef())
     );
   }
+
+  /**
+   * Returns true if the model has a static value (i.e. not a step) set for the given size dimension.
+   */
+  public hasStaticOuterDimension(dimension: LayoutSizeField) {
+    return this.size[dimension] && !isStep(this.size[dimension]);
+  }
 }
 
 /** Abstract class for UnitModel and FacetModel. Both of which can contain fieldDefs as a part of its own specification. */

src/compile/scale/range.ts

@@ -46,6 +46,7 @@ import {SignalRefWrapper} from '../signal';
 import {Explicit, makeExplicit, makeImplicit} from '../split';
 import {UnitModel} from '../unit';
 import {ScaleComponentIndex} from './component';
+import {isFacetModel} from "../model";
 
 export const RANGE_PROPERTIES: (keyof Scale)[] = ['range', 'scheme'];
 
@@ -203,12 +204,12 @@ function defaultRange(channel: ScaleChannel, model: UnitModel): VgRange {
       if (util.contains(['point', 'band'], scaleType)) {
         if (channel === X && !size.width) {
           const w = getViewConfigDiscreteSize(config.view, 'width');
-          if (isStep(w)) {
+          if (isStep(w) && (!isFacetModel(model.parent) || !model.parent.hasStaticOuterDimension('width'))) {
             return w;
           }
         } else if (channel === Y && !size.height) {
           const h = getViewConfigDiscreteSize(config.view, 'height');
-          if (isStep(h)) {
+          if (isStep(h) && (!isFacetModel(model.parent) || !model.parent.hasStaticOuterDimension('height'))) {
             return h;
           }
         }

src/spec/base.ts

@@ -98,6 +98,8 @@ export interface LayoutSizeMixins {
   height?: number | 'container' | Step; // Vega also supports SignalRef for width and height. However, we need to know if width is a step or not in VL and it's very difficult to check this at runtime, so we intentionally do not support SignalRef here.
 }
 
+export type LayoutSizeField = keyof LayoutSizeMixins;
+
 export function isFrameMixins(o: any): o is FrameMixins {
   return o['view'] || o['width'] || o['height'];
 }