handle degenerate domains

src/scales.js

@@ -425,10 +425,11 @@ function inferScaleType(key, channels, {type, domain, range, scheme, pivot, proj
   if (kind === opacity || kind === length) return "linear";
   if (kind === symbol) return "ordinal";
 
-  // If the domain or range has more than two values, assume it’s ordinal. You
-  // can still use a “piecewise” (or “polylinear”) scale, but you must set the
-  // type explicitly.
-  if ((domain || range || []).length > 2) return asOrdinalType(kind);
+  // If a domain or range is explicitly specified and doesn’t have two values,
+  // assume it’s ordinal. You can still use a “piecewise” (or “polylinear”)
+  // scale, but you must set the type explicitly.
+  const n = (domain ?? range)?.length;
+  if (n < 2 || n > 2) return asOrdinalType(kind);
 
   // Otherwise, infer the scale type from the data! Prefer the domain, if
   // present, over channels. (The domain and channels should be consistently

src/scales/quantitative.js

@@ -80,6 +80,7 @@ export function createScaleQ(
     reverse
   }
 ) {
+  domain = maybeRepeat(domain);
   interval = maybeRangeInterval(interval, type);
   if (type === "cyclical" || type === "sequential") type = "linear"; // shorthand for color schemes
   if (typeof interpolate !== "function") interpolate = maybeInterpolator(interpolate); // named interpolator
@@ -88,8 +89,8 @@ export function createScaleQ(
   // If an explicit range is specified, and it has a different length than the
   // domain, then redistribute the range using a piecewise interpolator.
   if (range !== undefined) {
-    const n = (domain = arrayify(domain)).length;
-    const m = (range = arrayify(range)).length;
+    const n = domain.length;
+    const m = (range = maybeRepeat(range)).length;
     if (n !== m) {
       if (interpolate.length === 1) throw new Error("invalid piecewise interpolator"); // e.g., turbo
       interpolate = piecewise(interpolate, range);
@@ -137,6 +138,11 @@ export function createScaleQ(
   return {type, domain, range, scale, interpolate, interval};
 }
 
+function maybeRepeat(values) {
+  values = arrayify(values);
+  return values.length >= 2 ? values : [values[0], values[0]];
+}
+
 function maybeNice(nice, type) {
   return nice === true ? undefined : typeof nice === "number" ? nice : maybeNiceInterval(nice, type);
 }

test/output/colorLegendDomainEmpty.html

@@ -0,0 +1,27 @@
+<div class="plot-swatches plot-swatches-wrap">
+  <style>
+    :where(.plot-swatches) {
+      font-family: system-ui, sans-serif;
+      font-size: 10px;
+      margin-bottom: 0.5em;
+    }
+
+    :where(.plot-swatch > svg) {
+      margin-right: 0.5em;
+      overflow: visible;
+    }
+
+    :where(.plot-swatches-wrap) {
+      display: flex;
+      align-items: center;
+      min-height: 33px;
+      flex-wrap: wrap;
+    }
+
+    :where(.plot-swatches-wrap .plot-swatch) {
+      display: inline-flex;
+      align-items: center;
+      margin-right: 1em;
+    }
+  </style>
+</div>

test/output/colorLegendDomainUnary.html

@@ -0,0 +1,29 @@
+<div class="plot-swatches plot-swatches-wrap">
+  <style>
+    :where(.plot-swatches) {
+      font-family: system-ui, sans-serif;
+      font-size: 10px;
+      margin-bottom: 0.5em;
+    }
+
+    :where(.plot-swatch > svg) {
+      margin-right: 0.5em;
+      overflow: visible;
+    }
+
+    :where(.plot-swatches-wrap) {
+      display: flex;
+      align-items: center;
+      min-height: 33px;
+      flex-wrap: wrap;
+    }
+
+    :where(.plot-swatches-wrap .plot-swatch) {
+      display: inline-flex;
+      align-items: center;
+      margin-right: 1em;
+    }
+  </style><span class="plot-swatch"><svg width="15" height="15" fill="#4269d0" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+      <rect width="100%" height="100%"></rect>
+    </svg>0</span>
+</div>

test/plots/legend-color.ts

@@ -34,6 +34,22 @@ export function colorLegendCategoricalReverse() {
   return Plot.legend({color: {domain: "ABCDEFGHIJ", reverse: true}});
 }
 
+export function colorLegendDomainUnary() {
+  return Plot.legend({color: {domain: [0]}});
+}
+
+export function colorLegendDomainEmpty() {
+  return Plot.legend({color: {domain: []}});
+}
+
+export function colorLegendLinearDomainUnary() {
+  return Plot.legend({color: {type: "linear", domain: [0]}});
+}
+
+export function colorLegendLinearDomainEmpty() {
+  return Plot.legend({color: {type: "linear", domain: []}});
+}
+
 export function colorLegendOrdinal() {
   return Plot.legend({color: {type: "ordinal", domain: "ABCDEFGHIJ"}});
 }