update README

Author: mbostock

README.md

@@ -422,6 +422,7 @@ The following *facet* constant options are also supported:
 * facet.**marginBottom** - the bottom margin
 * facet.**marginLeft** - the left margin
 * facet.**grid** - if true, draw grid lines for each facet
+* facet.**label** - if null, disable default facet axis labels
 
 Faceting can be explicitly enabled or disabled on a mark with the *facet* option, which accepts the following values:
 
@@ -450,7 +451,7 @@ When the *include* or *exclude* facet mode is chosen, the mark data must be para
 
 [Marks](https://observablehq.com/@observablehq/plot-marks) visualize data as geometric shapes such as bars, dots, and lines. An single mark can generate multiple shapes: for example, passing a [Plot.barY](#plotbarydata-options) to [Plot.plot](#plotplotoptions) will produce a bar for each element in the associated data. Multiple marks can be layered into [plots](#plotplotoptions).
 
-Mark constructors take two arguments: **data** and **options**. Together these describe a tabular dataset and how to visualize it. Options that are shared by all of a mark’s generated shapes are known as *constants*, while options that vary with the mark’s data are known as *channels*. Channels are typically bound to [scales](#scale-options) and encode abstract values, such as time or temperature, as visual values, such as position or color.
+Mark constructors take two arguments: **data** and **options**. Together these describe a tabular dataset and how to visualize it. Options that are shared by all of a mark’s generated shapes are known as *constants*, while options that vary with the mark’s data are known as *channels*. Channels are typically bound to [scales](#scale-options) and encode abstract values, such as time or temperature, as visual values, such as position or color. (Channels can also be used to order ordinal domains; see [sort options](#sort-options).)
 
 A mark’s data is most commonly an array of objects representing a tabular dataset, such as the result of loading a CSV file, while a mark’s options bind channels (such as *x* and *y*) to columns in the data (such as *units* and *fruit*).