the object opacity channel should opt-out of the opacity scale too (#1571)

Author: Fil

src/channel.js

@@ -51,6 +51,7 @@ export function inferChannelScale(name, channel) {
         break;
       case "fillOpacity":
       case "strokeOpacity":
+      case "opacity":
         channel.scale = scale !== true && isEvery(value, isOpacity) ? null : "opacity";
         break;
       case "symbol":

test/output/opacityDotsFillUnscaled.svg

@@ -164,6 +164,7 @@ export * from "./movies-rating-by-genre.js";
 export * from "./multiplication-table.js";
 export * from "./music-revenue.js";
 export * from "./npm-versions.js";
+export * from "./opacity.js";
 export * from "./ordinal-bar.js";
 export * from "./penguin-annotated.js";
 export * from "./penguin-culmen-array.js";

test/output/opacityDotsStrokeUnscaled.svg

@@ -0,0 +1,27 @@
+import * as d3 from "d3";
+import * as Plot from "@observablehq/plot";
+
+export function opacityDotsFillUnscaled() {
+  return Plot.dotX(d3.ticks(0.3, 0.7, 40), {
+    fill: "black",
+    r: 7,
+    fillOpacity: Plot.identity
+  }).plot();
+}
+
+export function opacityDotsStrokeUnscaled() {
+  return Plot.dotX(d3.ticks(0.3, 0.7, 40), {
+    stroke: "black",
+    r: 3.5,
+    strokeWidth: 7,
+    strokeOpacity: Plot.identity
+  }).plot();
+}
+
+export function opacityDotsUnscaled() {
+  return Plot.dotX(d3.ticks(0.3, 0.7, 40), {
+    fill: "black",
+    r: 7,
+    opacity: Plot.identity
+  }).plot();
+}