update README

Author: mbostock

README.md

@@ -54,7 +54,7 @@ Renders a new plot given the specified *options* and returns the corresponding S
 
 ### Mark options
 
-The **marks** option specifies an array of [marks](#marks) to render. Each mark has its own data and options; see the respective mark type (*e.g.*, [bar](#bar) or [dot](#dot)) for which mark options are supported. Each mark may be a nested array of marks, allowing composition. Marks may also be a function which returns an SVG element, if you wish to insert some arbitrary content into your plot. And marks may be null or undefined, which produce no output; this is useful for having marks shown conditionally (*e.g.*, when a box is checked). Marks are drawn in *z*-order, last on top. For example, here bars for the dataset *alphabet* are drawn on top of a single rule at *y* = 0.
+The **marks** option specifies an array of [marks](#marks) to render. Each mark has its own data and options; see the respective mark type (*e.g.*, [bar](#bar) or [dot](#dot)) for which mark options are supported. Each mark may be a nested array of marks, allowing composition. Marks may also be a function which returns an SVG element, if you wish to insert some arbitrary content into your plot. And marks may be null or undefined, which produce no output; this is useful for showing marks conditionally (*e.g.*, when a box is checked). Marks are drawn in *z*-order, last on top. For example, here bars for the dataset *alphabet* are drawn on top of a single rule at *y* = 0.
 
 ```js
 Plot.plot({
@@ -235,7 +235,7 @@ Plot automatically generates axes for position scales. You can configure these a
 * *scale*.**labelAnchor** - the label anchor: *top*, *right*, *bottom*, *left*, or *center*
 * *scale*.**labelOffset** - the label position offset (in pixels; default 0, typically for facet axes)
 
-Plot does not currently generate a legend for the *color*, *radius*, or *opacity* scales, but when it does, we expect that some of the above options will also be used to configure legends. Top-level options are also supported as shorthand: **grid** (for *x* and *y* only; see [facet.grid](#facet-options)), **inset**, **round**, **align**, and **padding**.
+Plot does not currently generate a legend for the *color*, *radius*, or *opacity* scales, but when it does, we expect that some of the above options will also be used to configure legends. Top-level options are also supported as shorthand: **grid** and **line** (for *x* and *y* only; see also [facet.grid](#facet-options)), **label**, **axis**, **inset**, **round**, **align**, and **padding**.
 
 ### Color options
 
@@ -396,6 +396,8 @@ If different sort options are needed for different ordinal scales, the channel n
 Plot.barY(alphabet, {x: "letter", y: "frequency", sort: {x: {value: "y", reverse: true}}})
 ```
 
+If the input channel is *data*, then the reducer is passed groups of the mark’s data; this is typically used in conjunction with a custom reducer function, as when the built-in single-channel reducers are insufficient.
+
 Note: when a string or a function, the sort option is interpreted as a [basic sort transform](#transforms). To use both sort options and a sort transform, use [Plot.sort](#plotsortorder-options).
 
 ### Facet options