expose a deduplicated ordinal domain

src/scales/ordinal.js

@@ -17,7 +17,7 @@ function createScaleO(key, scale, channels, {type, interval, domain, range, reve
   if (domain === undefined) domain = inferDomain(channels, interval, key);
   if (type === "categorical" || type === ordinalImplicit) type = "ordinal"; // shorthand for color schemes
   if (reverse) domain = reverseof(domain);
-  scale.domain(domain);
+  domain = scale.domain(domain).domain(); // deduplicate
   if (range !== undefined) {
     // If the range is specified as a function, pass it the domain.
     if (typeof range === "function") range = range(domain);

test/scales/scales-test.js

@@ -2102,6 +2102,36 @@ it("plot(…).scale(name).apply and invert return the expected functions", () =>
   ]);
 });
 
+it("plot(…).scale(name) returns a deduplicated ordinal domain", () => {
+  const letters = "abbbcaabbcc";
+  const plot = Plot.dotX(letters).plot({x: {domain: letters}});
+  scaleEqual(plot.scale("x"), {
+    align: 0.5,
+    bandwidth: 0,
+    domain: ["a", "b", "c"],
+    padding: 0.5,
+    range: [20, 620],
+    round: true,
+    step: 200,
+    type: "point"
+  });
+});
+
+it("plot(…).scale(name) returns a deduplicated ordinal/temporal domain", () => {
+  const dates = ["2001", "2002", "2004", "2004"].map(d3.isoParse);
+  const plot = Plot.dotX(dates).plot({x: {type: "point", domain: dates}});
+  scaleEqual(plot.scale("x"), {
+    align: 0.5,
+    bandwidth: 0,
+    domain: dates.slice(0, 3),
+    padding: 0.5,
+    range: [20, 620],
+    round: true,
+    step: 200,
+    type: "point"
+  });
+});
+
 // Given a plot specification (or, as shorthand, an array of marks or a single
 // mark), asserts that the given named scales, when materialized from the first
 // plot and used to produce a second plot, produce the same output and the same