Support for "nice" polylinear scales.

d3.js

@@ -2213,16 +2213,17 @@ d3.scale.linear = function() {
   };
 
   scale.nice = function() {
-    var start = domain[0],
-        end = domain[domain.length - 1],
+    var last = domain.length - 1,
+        start = domain[0],
+        end = domain[last],
         reverse = end < start,
         min = reverse ? end : start,
         max = reverse ? start : end,
         span = max - min;
 
     var step = Math.pow(10, Math.round(Math.log(span) / Math.log(10)) - 1);
-    domain = [Math.floor(min / step) * step, Math.ceil(max / step) * step];
-    if (reverse) domain.reverse();
+    domain[reverse ? last : 0] = Math.floor(min / step) * step;
+    domain[reverse ? 0 : last] = Math.ceil(max / step) * step;
 
     return rescale();
   };

src/scale/linear.js

@@ -83,16 +83,17 @@ d3.scale.linear = function() {
   };
 
   scale.nice = function() {
-    var start = domain[0],
-        end = domain[domain.length - 1],
+    var last = domain.length - 1,
+        start = domain[0],
+        end = domain[last],
         reverse = end < start,
         min = reverse ? end : start,
         max = reverse ? start : end,
         span = max - min;
 
     var step = Math.pow(10, Math.round(Math.log(span) / Math.log(10)) - 1);
-    domain = [Math.floor(min / step) * step, Math.ceil(max / step) * step];
-    if (reverse) domain.reverse();
+    domain[reverse ? last : 0] = Math.floor(min / step) * step;
+    domain[reverse ? 0 : last] = Math.ceil(max / step) * step;
 
     return rescale();
   };

tests/test-scale-polylinear.js

@@ -28,3 +28,10 @@ console.log("          0.5 -> " + f(x(.5)));
 console.log("          1.0 -> " + f(x(1)));
 console.log("          1.5 -> " + f(x(1.5)));
 console.log("");
+
+var x = d3.scale.linear();
+[[1.1, 1, 2, 3, 10.9], [123.1, 1, 2, 3, -.9]].forEach(function(d) {
+  console.log("domain([" + d.map(f) + " ]).nice():");
+  console.log("  ", x.domain(d).nice().domain());
+  console.log("");
+});

tests/test-scale-polylinear.out

@@ -20,3 +20,9 @@ domain(-1, 0, 1).range(-100, 0, 10):
           1.0 ->  10.000
           1.5 ->  15.000
 
+domain([ 1.100, 1.000, 2.000, 3.000, 10.900 ]).nice():
+   [ 1, 1, 2, 3, 11 ]
+
+domain([ 123.100, 1.000, 2.000, 3.000,−0.900 ]).nice():
+   [ 130, 1, 2, 3, -10 ]
+