fix tickFormat type inference for empty domain

src/marks/axis.js

@@ -672,10 +672,10 @@ export function inferTickFormat(scale, data, ticks, tickFormat, anchor) {
     ? inferTimeFormat(scale.type, data, anchor) ?? formatDefault
     : scale.tickFormat
     ? scale.tickFormat(typeof ticks === "number" ? ticks : null, tickFormat)
+    : typeof tickFormat === "string" && scale.domain().length > 0
+    ? (isTemporal(scale.domain()) ? utcFormat : format)(tickFormat)
     : tickFormat === undefined
     ? formatDefault
-    : typeof tickFormat === "string"
-    ? (isTemporal(scale.domain()) ? utcFormat : format)(tickFormat)
     : constant(tickFormat);
 }
 

src/scales.js

@@ -425,10 +425,10 @@ 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 the domain or range doesn’t have exactly 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);
 
   // Otherwise, infer the scale type from the data! Prefer the domain, if
   // present, over channels. (The domain and channels should be consistently

test/plots/index.ts

@@ -308,6 +308,7 @@ export * from "./style-overrides.js";
 export * from "./symbol-set.js";
 export * from "./text-overflow.js";
 export * from "./this-is-just-to-say.js";
+export * from "./tick-format.js";
 export * from "./time-axis.js";
 export * from "./tip-format.js";
 export * from "./tip.js";

test/plots/tick-format.ts

@@ -0,0 +1,5 @@
+import * as Plot from "@observablehq/plot";
+
+export async function tickFormatEmptyDomain() {
+  return Plot.plot({y: {tickFormat: "%W"}, marks: [Plot.dotX([0]), Plot.barY([]), Plot.frame()]});
+}