The plot's scales object contains the minimum set of options that can be used to recreate the scale in another plot. In particular it doesn't show the (D3) scale anymore. Instead, a convenience function Plot.scale(options) is provided, that will turn a scale *options* object into a D3 scale, ready to use in another context. (This also clears the question of mutability.)

Author: Fil

README.md

@@ -200,21 +200,24 @@ Plot.plot({
 })
 ```
 
-All the scales are exposed as the *scales* property of the plot. We recommend using *scale*.copy if making any use of these scales that might mutate them. 
+All the scale definitions are exposed as the *scales* property of the plot.
 
 ```js
 color = Plot.plot({…}).scales.color;
-color.range() // ["red", "blue"]
+color.range // ["red", "blue"]
 ```
 
 And, to reuse the scale in another plot:
 
 ```js
+const plot1 = Plot.plot(…);
+
 Plot.plot({
-  color: { domain: color.copy().domain(), range: color.copy().range() }
+  color: plot:.scales.color
 })
 ```
 
+Plot.*scale*(*options*) returns a [D3 scale](https://github.com/d3/d3-scale) that matches the Plot *options* object.
 
 ### Position options
 

src/index.js

@@ -1,5 +1,6 @@
 export {plot} from "./plot.js";
 export {Mark, valueof} from "./mark.js";
+export {scale} from "./scales.js";
 export {Area, area, areaX, areaY} from "./marks/area.js";
 export {BarX, BarY, barX, barY} from "./marks/bar.js";
 export {Cell, cell, cellX, cellY} from "./marks/cell.js";

src/scales.js

@@ -82,6 +82,10 @@ function Scale(key, channels = [], options = {}) {
   return scale;
 }
 
+export function scale(options) {
+  return Scale(undefined, undefined, options).scale;
+}
+
 function inferScaleType(key, channels, {type, domain, range}) {
   if (key === "fx" || key === "fy") return "band";
   if (type !== undefined) {
@@ -128,7 +132,6 @@ function exposeScale({scale, label}) {
     ...scale.interpolate && {interpolate: scale.interpolate()},
     ...label !== undefined && {label},
     ...scale.type && {type: scale.type},
-    ...scale.clamp && scale.clamp() && {clamp: true},
-    scale
+    ...scale.clamp && scale.clamp() && {clamp: true}
   };
 }