fix inversion when collapsed domain (#2027)

src/scales/quantitative.js

@@ -123,8 +123,9 @@ export function createScaleQ(
     const [min, max] = extent(domain);
     if (min > 0 || max < 0) {
       domain = slice(domain);
-      if (orderof(domain) !== Math.sign(min)) domain[domain.length - 1] = 0; // [2, 1] or [-2, -1]
-      else domain[0] = 0; // [1, 2] or [-1, -2]
+      const o = orderof(domain) || 1; // treat degenerate as ascending
+      if (o === Math.sign(min)) domain[0] = 0; // [1, 2] or [-1, -2]
+      else domain[domain.length - 1] = 0; // [2, 1] or [-2, -1]
     }
   }
 

test/plots/index.ts

@@ -340,3 +340,4 @@ export * from "./yearly-requests-dot.js";
 export * from "./yearly-requests-line.js";
 export * from "./yearly-requests.js";
 export * from "./young-adults.js";
+export * from "./zero.js";

test/plots/zero.ts

@@ -0,0 +1,17 @@
+import * as Plot from "@observablehq/plot";
+
+export async function zeroNegativeY() {
+  return Plot.lineY([-0.25, -0.15, -0.05]).plot({y: {zero: true}});
+}
+
+export async function zeroPositiveY() {
+  return Plot.lineY([0.25, 0.15, 0.05]).plot({y: {zero: true}});
+}
+
+export async function zeroPositiveDegenerateY() {
+  return Plot.lineY([0.25, 0.25, 0.25]).plot({y: {zero: true}});
+}
+
+export async function zeroNegativeDegenerateY() {
+  return Plot.lineY([-0.25, -0.25, -0.25]).plot({y: {zero: true}});
+}