Fix ordinal scales with singleton domains.

Fixes #1717. Turns out, -1 % 1 is 0!

d3.js

@@ -7615,7 +7615,7 @@
   function d3_scale_ordinal(domain, ranger) {
     var index, range, rangeBand;
     function scale(x) {
-      return range[((index.get(x) || ranger.t === "range" && index.set(x, domain.push(x))) - 1) % range.length];
+      return range[((index.get(x) || (ranger.t === "range" ? index.set(x, domain.push(x)) : NaN)) - 1) % range.length];
     }
     function steps(start, step) {
       return d3.range(domain.length).map(function(i) {

src/scale/ordinal.js

@@ -12,7 +12,7 @@ function d3_scale_ordinal(domain, ranger) {
       rangeBand;
 
   function scale(x) {
-    return range[((index.get(x) || ranger.t === "range" && index.set(x, domain.push(x))) - 1) % range.length];
+    return range[((index.get(x) || (ranger.t === "range" ? index.set(x, domain.push(x)) : NaN)) - 1) % range.length];
   }
 
   function steps(start, step) {

test/scale/ordinal-test.js

@@ -117,6 +117,8 @@ suite.addBatch({
       "correctly handles singleton domains": function(ordinal) {
         var x = ordinal().domain(["a"]).rangePoints([0, 120]);
         assert.deepEqual(x.range(), [60]);
+        assert.isUndefined(x("b"));
+        assert.deepEqual(x.domain(), ["a"]);
       },
       "can be set to a descending range": function(ordinal) {
         var x = ordinal().domain(["a", "b", "c"]).rangePoints([120, 0]);