Saturation is only undefined for black and white.

d3.js

@@ -1442,11 +1442,14 @@ d3 = function() {
     return rgb(r, g, b);
   }
   function d3_rgb_hsl(r, g, b) {
-    var min = Math.min(r /= 255, g /= 255, b /= 255), max = Math.max(r, g, b), d = max - min, h = NaN, s = h, l = (max + min) / 2;
+    var min = Math.min(r /= 255, g /= 255, b /= 255), max = Math.max(r, g, b), d = max - min, h, s, l = (max + min) / 2;
     if (d) {
       s = l < .5 ? d / (max + min) : d / (2 - max - min);
       if (r == max) h = (g - b) / d + (g < b ? 6 : 0); else if (g == max) h = (b - r) / d + 2; else h = (r - g) / d + 4;
       h *= 60;
+    } else {
+      h = NaN;
+      s = l > 0 && l < 1 ? 0 : h;
     }
     return d3_hsl(h, s, l);
   }

src/color/rgb.js

@@ -117,8 +117,8 @@ function d3_rgb_hsl(r, g, b) {
   var min = Math.min(r /= 255, g /= 255, b /= 255),
       max = Math.max(r, g, b),
       d = max - min,
-      h = NaN,
-      s = h,
+      h,
+      s,
       l = (max + min) / 2;
   if (d) {
     s = l < .5 ? d / (max + min) : d / (2 - max - min);
@@ -126,6 +126,9 @@ function d3_rgb_hsl(r, g, b) {
     else if (g == max) h = (b - r) / d + 2;
     else h = (r - g) / d + 4;
     h *= 60;
+  } else {
+    h = NaN;
+    s = l > 0 && l < 1 ? 0 : h;
   }
   return d3_hsl(h, s, l);
 }

test/color/hsl-test.js

@@ -95,6 +95,10 @@ suite.addBatch({
       assert.strictEqual(hsl(0, .42, 1).s, .42);
       assert.strictEqual(hsl(0, 1, 1).s, 1);
     },
+    "s is zero for grayscale colors (but not white and black)": function(hsl) {
+      assert.strictEqual(hsl("#ccc").s, 0);
+      assert.strictEqual(hsl("#777").s, 0);
+    },
     "s is undefined when not explicitly specified for white or black": function(hsl) {
       assert.isNaN(hsl("#000").s);
       assert.isNaN(hsl("black").s);