Treat null as undefined.

src/continuous.js

@@ -83,7 +83,7 @@ export function transformer() {
   }
 
   function scale(x) {
-    return isNaN(x = +x) ? unknown : (output || (output = piecewise(domain.map(transform), range, interpolate)))(transform(clamp(x)));
+    return x == null || isNaN(x = +x) ? unknown : (output || (output = piecewise(domain.map(transform), range, interpolate)))(transform(clamp(x)));
   }
 
   scale.invert = function(y) {

src/identity.js

@@ -5,7 +5,7 @@ export default function identity(domain) {
   var unknown;
 
   function scale(x) {
-    return isNaN(x = +x) ? unknown : x;
+    return x == null || isNaN(x = +x) ? unknown : x;
   }
 
   scale.invert = scale;

src/quantile.js

@@ -15,7 +15,7 @@ export default function quantile() {
   }
 
   function scale(x) {
-    return isNaN(x = +x) ? unknown : range[bisect(thresholds, x)];
+    return x == null || isNaN(x = +x) ? unknown : range[bisect(thresholds, x)];
   }
 
   scale.invertExtent = function(y) {

src/quantize.js

@@ -11,7 +11,7 @@ export default function quantize() {
       unknown;
 
   function scale(x) {
-    return x <= x ? range[bisect(domain, x, 0, n)] : unknown;
+    return x != null && x <= x ? range[bisect(domain, x, 0, n)] : unknown;
   }
 
   function rescale() {

src/sequential.js

@@ -18,7 +18,7 @@ function transformer() {
       unknown;
 
   function scale(x) {
-    return isNaN(x = +x) ? unknown : interpolator(k10 === 0 ? 0.5 : (x = (transform(x) - t0) * k10, clamp ? Math.max(0, Math.min(1, x)) : x));
+    return x == null || isNaN(x = +x) ? unknown : interpolator(k10 === 0 ? 0.5 : (x = (transform(x) - t0) * k10, clamp ? Math.max(0, Math.min(1, x)) : x));
   }
 
   scale.domain = function(_) {

src/sequentialQuantile.js

@@ -7,7 +7,7 @@ export default function sequentialQuantile() {
       interpolator = identity;
 
   function scale(x) {
-    if (!isNaN(x = +x)) return interpolator((bisect(domain, x, 1) - 1) / (domain.length - 1));
+    if (x != null && !isNaN(x = +x)) return interpolator((bisect(domain, x, 1) - 1) / (domain.length - 1));
   }
 
   scale.domain = function(_) {

src/threshold.js

@@ -8,7 +8,7 @@ export default function threshold() {
       n = 1;
 
   function scale(x) {
-    return x <= x ? range[bisect(domain, x, 0, n)] : unknown;
+    return x != null && x <= x ? range[bisect(domain, x, 0, n)] : unknown;
   }
 
   scale.domain = function(_) {

test/identity-test.js

@@ -31,6 +31,16 @@ tape("identity(x) coerces input to a number", function(test) {
   test.end();
 });
 
+tape("identity(undefined) returns unknown", function(test) {
+  var s = scale.scaleIdentity().unknown(-1);
+  test.equal(s(undefined), -1);
+  test.equal(s(null), -1);
+  test.equal(s(NaN), -1);
+  test.equal(s("N/A"), -1);
+  test.equal(s(0.4), 0.4);
+  test.end();
+});
+
 tape("identity.invert(y) is the identity function", function(test) {
   var s = scale.scaleIdentity().domain([1, 2]);
   test.equal(s.invert(0.5), 0.5);

test/linear-test.js

@@ -209,8 +209,9 @@ tape("linear.rangeRound(range) accepts an iterable", function(test) {
   test.end();
 });
 
-tape("linear.unknown(value) sets the return value for undefined and NaN input", function(test) {
+tape("linear.unknown(value) sets the return value for undefined, null, and NaN input", function(test) {
   var s = scale.scaleLinear().unknown(-1);
+  test.equal(s(null), -1);
   test.equal(s(undefined), -1);
   test.equal(s(NaN), -1);
   test.equal(s("N/A"), -1);
@@ -388,15 +389,12 @@ tape("linear.ticks(X) spans linear.nice(X).domain()", function(test) {
     var ticks = s.ticks(count);
     test.deepEqual([ticks[0], ticks[ticks.length - 1]], s.domain());
   }
-
   check([1, 9], 2);
   check([1, 9], 3);
   check([1, 9], 4);
-
   check([8, 9], 2);
   check([8, 9], 3);
   check([8, 9], 4);
-
   check([1, 21], 2);
   check([2, 21], 2);
   check([3, 21], 2);
@@ -408,7 +406,6 @@ tape("linear.ticks(X) spans linear.nice(X).domain()", function(test) {
   check([9, 21], 2);
   check([10, 21], 2);
   check([11, 21], 2);
-
   test.end();
 })
 

test/quantile-test.js

@@ -129,9 +129,10 @@ tape("quantile.invertExtent() returns the first match if duplicate values exist
   test.end();
 });
 
-tape("quantile.unknown(value) sets the return value for undefined and NaN input", function(test) {
+tape("quantile.unknown(value) sets the return value for undefined, null, and NaN input", function(test) {
   var s = scale.scaleQuantile().domain([3, 6, 7, 8, 8, 10, 13, 15, 16, 20]).range([0, 1, 2, 3]).unknown(-1);
   test.equal(s(undefined), -1);
+  test.equal(s(null), -1);
   test.equal(s(NaN), -1);
   test.equal(s("N/A"), -1);
   test.equal(s(2), 0);

test/quantize-test.js

@@ -36,6 +36,14 @@ tape("quantize(value) clamps input values to the domain", function(test) {
   test.end();
 });
 
+tape("quantize.unknown(value) sets the return value for undefined, null, and NaN input", function(test) {
+  var s = scale.scaleQuantize().range([0, 1, 2]).unknown(-1);
+  test.equal(s(undefined), -1);
+  test.equal(s(null), -1);
+  test.equal(s(NaN), -1);
+  test.end();
+});
+
 tape("quantize.domain() coerces domain values to numbers", function(test) {
   var s = scale.scaleQuantize().domain(["-1.20", "2.40"]);
   test.deepEqual(s.domain(), [-1.2, 2.4]);

test/threshold-test.js

@@ -26,7 +26,7 @@ tape("threshold(x) returns undefined if the specified value x is not orderable",
   test.equal(x(), undefined);
   test.equal(x(undefined), undefined);
   test.equal(x(NaN), undefined);
-  test.equal(x(null), "a"); // null < 1/3
+  test.equal(x(null), undefined);
   test.end();
 });