Update README

Author: mbostock

README.md

@@ -512,15 +512,15 @@ Returns a new area with the given *data* and *options*. Plot.area is rarely used
 Plot.areaX(aapl, {y: "Date", x: "Close"})
 ```
 
-Equivalent to [Plot.area](#plotareadata-options), except that the *y* option specifies the *y1* channel, and if the *x* option is specified, it corresponds to the *x2* channel while the *x1* channel defaults to zero. This constructor is typically used for vertically-oriented area charts (*e.g.*, when time goes up↑).
+Returns a new area with the given *data* and *options*. This constructor is used when the baseline and topline share *y* values, as in a time-series area chart where time goes up↑. If neither the **x1** nor **x2** option is specified, a **x** option may be specified as shorthand to apply an implicit [stackX transform](#plotstackxoptions); this is the typical configuration for an area chart with a baseline at *x* = 0. If the **x** option is not specified, it defaults to the identity function. The **y** option specifies the **y1** channel; and the **y1** and **y2** options are ignored.
 
 #### Plot.areaY(*data*, *options*)
 
 ```js
 Plot.areaY(aapl, {x: "Date", y: "Close"})
 ```
 
-Equivalent to [Plot.area](#plotareadata-options), except that the *x* option specifies the *x1* channel, and if the *y* option is specified, it corresponds to the *y2* channel while the *y1* channel defaults to zero. This constructor is typically used for horizontally-oriented area charts (*e.g.*, when time goes right→).
+Returns a new area with the given *data* and *options*. This constructor is used when the baseline and topline share *x* values, as in a time-series area chart where time goes right→. If neither the **y1** nor **y2** option is specified, a **y** option may be specified as shorthand to apply an implicit [stackY transform](#plotstackyoptions); this is the typical configuration for an area chart with a baseline at *y* = 0. If the **y** option is not specified, it defaults to the identity function. The **x** option specifies the **x1** channel; and the **x1** and **x2** options are ignored.
 
 ### Bar