document the {value, scale} pattern for color and symbol channels

README.md

@@ -161,7 +161,7 @@ Plot.plot({
 
 Plot supports many scale types. Some scale types are for quantitative data: values that can be added or subtracted, such as temperature or time. Other scale types are for ordinal or categorical data: unquantifiable values that can only be ordered, such as t-shirt sizes, or values with no inherent order that can only be tested for equality, such as types of fruit. Some scale types are further intended for specific visual encodings: for example, as [position](#position-options) or [color](#color-options).
 
-You can set the scale type explicitly via the *scale*.**type** option, though typically the scale type is inferred automatically. Some marks mandate a particular scale type: for example, [Plot.barY](#plotbarydata-options) requires that the *x* scale is a *band* scale. Some scales have a default type: for example, the *radius* scale defaults to *sqrt* and the *opacity* scale defaults to *linear*. Most often, the scale type is inferred from associated data, pulled either from the domain (if specified) or from associated channels. A *color* scale defaults to *identity* if no range or scheme is specified and all associated defined values are valid CSS color strings. Otherwise, strings and booleans imply an ordinal scale; dates imply a UTC scale; and anything else is linear. Unless they represent text, we recommend explicitly converting strings to more specific types when loading data (*e.g.*, with d3.autoType or Observable’s FileAttachment). For simplicity’s sake, Plot assumes that data is consistently typed; type inference is based solely on the first non-null, non-undefined value.
+You can set the scale type explicitly via the *scale*.**type** option, though typically the scale type is inferred automatically. Some marks mandate a particular scale type: for example, [Plot.barY](#plotbarydata-options) requires that the *x* scale is a *band* scale. Some scales have a default type: for example, the *radius* scale defaults to *sqrt* and the *opacity* scale defaults to *linear*. Most often, the scale type is inferred from associated data, pulled either from the domain (if specified) or from associated channels. Strings and booleans imply an ordinal scale; dates imply a UTC scale; and anything else is linear. Unless they represent text, we recommend explicitly converting strings to more specific types when loading data (*e.g.*, with d3.autoType or Observable’s FileAttachment). For simplicity’s sake, Plot assumes that data is consistently typed; type inference is based solely on the first non-null, non-undefined value.
 
 For quantitative data (*i.e.* numbers), a mathematical transform may be applied to the data by changing the scale type:
 
@@ -800,6 +800,20 @@ All marks support the following optional channels:
 
 The **fill**, **fillOpacity**, **stroke**, **strokeWidth**, **strokeOpacity**, and **opacity** options can be specified as either channels or constants. When the fill or stroke is specified as a function or array, it is interpreted as a channel; when the fill or stroke is specified as a string, it is interpreted as a constant if a valid CSS color and otherwise it is interpreted as a column name for a channel. Similarly when the fill opacity, stroke opacity, object opacity, stroke width, or radius is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel.
 
+The scale associated with any channel can be overridden by specifying the channel as an object with a *value* property specifying the channel values and a *scale* property specifying the desired scale name or null for an unscaled channel. For example, to force the **stroke** channel to be unscaled, interpreting the associated values as literal color strings:
+
+```js
+Plot.dot(data, {stroke: {value: "fieldName", scale: null}})
+```
+
+To instead force the **stroke** channel to be bound to the *color* scale regardless of the provided values, say:
+
+```js
+Plot.dot(data, {stroke: {value: "fieldName", scale: "color"}})
+```
+
+The color channels (**fill** and **stroke**) are bound to the *color* scale by default, unless the provided values are all valid CSS color strings or nullish, in which case the values are interpreted literally and unscaled.
+
 The **title**, **href**, and **ariaLabel** options can *only* be specified as channels. When these options are specified as a string, the string refers to the name of a column in the mark’s associated data. If you’d like every instance of a particular mark to have the same value, specify the option as a function that returns the desired value, *e.g.* `() => "Hello, world!"`.
 
 The rectangular marks ([bar](#bar), [cell](#cell), [frame](#frame), and [rect](#rect)) support insets and rounded corner constant options:
@@ -1461,7 +1475,7 @@ The following dot-specific constant options are also supported:
 
 The **r** option can be specified as either a channel or constant. When the radius is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel. The radius defaults to 4.5 pixels when using the **symbol** channel, and otherwise 3 pixels. Dots with a nonpositive radius are not drawn.
 
-The **stroke** defaults to none. The **fill** defaults to currentColor if the stroke is none, and to none otherwise. The **strokeWidth** defaults to 1.5. The **rotate** and **symbol** options can be specified as either channels or constants. When rotate is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel. When symbol is a valid symbol name or symbol object (implementing the draw method), it is interpreted as a constant; otherwise it is interpreted as a channel.
+The **stroke** defaults to none. The **fill** defaults to currentColor if the stroke is none, and to none otherwise. The **strokeWidth** defaults to 1.5. The **rotate** and **symbol** options can be specified as either channels or constants. When rotate is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel. When symbol is a valid symbol name or symbol object (implementing the draw method), it is interpreted as a constant; otherwise it is interpreted as a channel. If the **symbol** channel’s values are all symbols, symbol names, or nullish, the channel is unscaled (values are interpreted literally); otherwise, the channel is bound to the *symbol* scale.
 
 The built-in **symbol** types are: *circle*, *cross*, *diamond*, *square*, *star*, *triangle*, and *wye* (for fill) and *circle*, *plus*, *times*, *triangle2*, *asterisk*, *square2*, and *diamond2* (for stroke, based on [Heman Robinson’s research](https://www.tandfonline.com/doi/abs/10.1080/10618600.2019.1637746)). The *hexagon* symbol is also supported. You can also specify a D3 or custom symbol type as an object that implements the [*symbol*.draw(*context*, *size*)](https://github.com/d3/d3-shape/blob/main/README.md#custom-symbol-types) method.