Add d3.scale.identity. Fixes #483.

Makefile

@@ -129,7 +129,8 @@ d3.scale.js: \
 	src/scale/ordinal.js \
 	src/scale/category.js \
 	src/scale/quantile.js \
-	src/scale/quantize.js
+	src/scale/quantize.js \
+	src/scale/identity.js
 
 d3.svg.js: \
 	src/svg/svg.js \

d3.v2.js

@@ -2962,6 +2962,36 @@ function d3_scale_quantize(x0, x1, range) {
 
   return rescale();
 }
+d3.scale.identity = function() {
+  return d3_scale_identity([0, 1]);
+};
+
+function d3_scale_identity(domain) {
+
+  function identity(x) { return x; }
+
+  identity.invert = identity;
+
+  identity.domain = identity.range = function(x) {
+    if (!arguments.length) return domain;
+    domain = x;
+    return identity;
+  };
+
+  identity.ticks = function(m) {
+    return d3_scale_linearTicks(domain, m);
+  };
+
+  identity.tickFormat = function(m) {
+    return d3_scale_linearTickFormat(domain, m);
+  };
+
+  identity.copy = function() {
+    return d3_scale_identity(domain);
+  };
+
+  return identity;
+}
 d3.svg = {};
 d3.svg.arc = function() {
   var innerRadius = d3_svg_arcInnerRadius,

src/scale/identity.js

@@ -0,0 +1,30 @@
+d3.scale.identity = function() {
+  return d3_scale_identity([0, 1]);
+};
+
+function d3_scale_identity(domain) {
+
+  function identity(x) { return x; }
+
+  identity.invert = identity;
+
+  identity.domain = identity.range = function(x) {
+    if (!arguments.length) return domain;
+    domain = x;
+    return identity;
+  };
+
+  identity.ticks = function(m) {
+    return d3_scale_linearTicks(domain, m);
+  };
+
+  identity.tickFormat = function(m) {
+    return d3_scale_linearTickFormat(domain, m);
+  };
+
+  identity.copy = function() {
+    return d3_scale_identity(domain);
+  };
+
+  return identity;
+}