Update README

Author: mbostock

README.md

@@ -1132,19 +1132,19 @@ Groups on the first channel of *z*, *fill*, or *stroke*, if any. If none of *z*,
 
 [<img src="./img/window.png" width="320" height="198" alt="moving averages of daily highs and lows">](https://observablehq.com/@data-workflows/plot-map)
 
-[Source](./src/transforms/map.js) · [Examples](https://observablehq.com/@data-workflows/plot-map) · Groups data into series along the *z* dimension, and then applies a mapping function to the values for each series, say to normalize them relative to some basis or to apply a moving average.
+[Source](./src/transforms/map.js) · [Examples](https://observablehq.com/@data-workflows/plot-map) · Groups data into series along the *z* dimension and then applies a mapping function to each series’ values, say to normalize them relative to some basis or to apply a moving average.
 
-The map transform derives new output channels from corresponding input channels. The output channels have the same length as the input channels; the map transform does not affect the mark’s data or index. The map transform is similar to running [*array*.map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) on the input channel’s values with the given function; however, the map transform is series-aware: the data is first grouped into series along the *z* dimension in the same fashion as the [area](#area) and [line](#line) marks so that series are processed independently. (You wouldn’t want a moving average to bleed between series, would you?)
+The map transform derives new output channels from corresponding input channels. The output channels have strictly the same length as the input channels; the map transform does not affect the mark’s data or index. The map transform is similar to running [*array*.map](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map) on the input channel’s values with the given function; however, the map transform is series-aware: the data are first grouped into series along the *z* dimension in the same fashion as the [area](#area) and [line](#line) marks so that series can be processed independently. (You wouldn’t want a moving average to bleed between series, right?)
 
-Like the [group](#group) and [bin](#bin) transforms, the base [Plot.map](#plotmapoutputs-options) takes two arguments: an *outputs* object that describes the output channels to compute, and an *options* object that describes the input channels and additional options to propagate. The other map transforms, such as [Plot.normalizeX](#plotnormalizexoptions) and [Plot.windowX](#plotwindowxoptions), call Plot.map internally.
+Like the [group](#group) and [bin](#bin) transforms, the [Plot.map](#plotmapoutputs-options) transform takes two arguments: an *outputs* object that describes the output channels to compute, and an *options* object that describes the input channels and additional options to propagate. The other map transforms, such as [Plot.normalizeX](#plotnormalizexoptions) and [Plot.windowX](#plotwindowxoptions), call Plot.map internally.
 
 The following map methods are supported:
 
 * *cumsum* - a cumulative sum
 * a function to be passed an array of values, returning new values
 * an object that implements the *map* method
 
-The *map* method is repeatedly passed the index for each series (an array of integers), the corresponding input channel’s array of values, and the output channel’s array of values; it must populate the slots specified by the index in the output array.
+If a function is used, it must return an array of the same length as the given input. If a *map* method is used, it is repeatedly passed the index for each series (an array of integers), the corresponding input channel’s array of values, and the output channel’s array of values; it must populate the slots specified by the index in the output array.
 
 The Plot.normalizeX and Plot.normalizeY transforms take an additional **basis** option which specifies how to normalize the series values. The following normalization methods are supported: