Add d3.gray.

README.md

@@ -119,6 +119,10 @@ Constructs a new [Lab](https://en.wikipedia.org/wiki/Lab_color_space#CIELAB) col
 
 If *l*, *a* and *b* are specified, these represent the channel values of the returned color; an *opacity* may also be specified. If a CSS Color Module Level 3 *specifier* string is specified, it is parsed and then converted to the Lab color space. See [color](#color) for examples. If a [*color*](#color) instance is specified, it is converted to the RGB color space using [*color*.rgb](#color_rgb) and then converted to Lab. (Colors already in the Lab color space skip the conversion to RGB, and colors in the HCL color space are converted directly to Lab.)
 
+<a name="gray" href="#gray">#</a> d3.<b>gray</b>(<i>l</i>[, <i>opacity</i>]) [<>](https://github.com/d3/d3-color/blob/master/src/lab.js "Source")<br>
+
+Constructs a new [Lab](#lab) color with the specified *l* value and *a* = *b* = 0.
+
 <a name="hcl" href="#hcl">#</a> d3.<b>hcl</b>(<i>h</i>, <i>c</i>, <i>l</i>[, <i>opacity</i>]) [<>](https://github.com/d3/d3-color/blob/master/src/lab.js "Source")<br>
 <a href="#hcl">#</a> d3.<b>hcl</b>(<i>specifier</i>)<br>
 <a href="#hcl">#</a> d3.<b>hcl</b>(<i>color</i>)<br>

index.js

@@ -1,3 +1,3 @@
 export {default as color, rgb, hsl} from "./src/color";
-export {default as lab, hcl, lch} from "./src/lab";
+export {default as lab, hcl, lch, gray} from "./src/lab";
 export {default as cubehelix} from "./src/cubehelix";

src/lab.js

@@ -31,6 +31,10 @@ function labConvert(o) {
   return new Lab(116 * y - 16, 500 * (x - y), 200 * (y - z), o.opacity);
 }
 
+export function gray(l, opacity) {
+  return new Lab(l, 0, 0, opacity == null ? 1 : opacity);
+}
+
 export default function lab(l, a, b, opacity) {
   return arguments.length === 1 ? labConvert(l) : new Lab(l, a, b, opacity == null ? 1 : opacity);
 }

test/gray-test.js

@@ -0,0 +1,12 @@
+var tape = require("tape"),
+    color = require("../");
+
+require("./labEqual");
+
+tape("gray(l[, opacity]) is an alias for lab(l, 0, 0[, opacity])", function(test) {
+  test.labEqual(color.gray(120), 120, 0, 0, 1);
+  test.labEqual(color.gray(120, 0.5), 120, 0, 0, 0.5);
+  test.labEqual(color.gray(120, null), 120, 0, 0, 1);
+  test.labEqual(color.gray(120, undefined), 120, 0, 0, 1);
+  test.end();
+});