Consistent SI-prefix for ticks. Fixes #1746.

When a SI-prefix format (type "s") is passed to scale.tickFormat, compute a
suitable SI-prefix based on the maximum value in the range, and then use that
prefix for all ticks rather than computing the SI-prefix on a per-tick basis.

d3.js

@@ -7471,9 +7471,23 @@
   }
   function d3_scale_linearTickFormat(domain, m, format) {
     var range = d3_scale_linearTickRange(domain, m);
-    return d3.format(format ? format.replace(d3_format_re, function(a, b, c, d, e, f, g, h, i, j) {
-      return [ b, c, d, e, f, g, h, i || "." + d3_scale_linearFormatPrecision(j, range), j ].join("");
-    }) : ",." + d3_scale_linearPrecision(range[2]) + "f");
+    if (format) {
+      var match = d3_format_re.exec(format);
+      match.shift();
+      if (match[8] === "s") {
+        var prefix = d3.formatPrefix(Math.max(abs(range[0]), abs(range[1])) * (match[7] ? Math.pow(10, -match[7].substring(1)) : 1));
+        match[7] = "." + d3_scale_linearPrecision(prefix.scale(range[2])), match[8] = "f";
+        format = d3.format(match.join(""));
+        return function(d) {
+          return format(prefix.scale(d)) + prefix.symbol;
+        };
+      }
+      if (!match[7]) match[7] = "." + d3_scale_linearFormatPrecision(match[8], range);
+      format = match.join("");
+    } else {
+      format = ",." + d3_scale_linearPrecision(range[2]) + "f";
+    }
+    return d3.format(format);
   }
   var d3_scale_linearFormatSignificant = {
     s: 1,

src/scale/linear.js

@@ -4,6 +4,7 @@ import "../interpolate/interpolate";
 import "../interpolate/round";
 import "../interpolate/uninterpolate";
 import "../format/format";
+import "../math/abs";
 import "bilinear";
 import "nice";
 import "polylinear";
@@ -116,9 +117,23 @@ function d3_scale_linearTicks(domain, m) {
 
 function d3_scale_linearTickFormat(domain, m, format) {
   var range = d3_scale_linearTickRange(domain, m);
-  return d3.format(format
-      ? format.replace(d3_format_re, function(a, b, c, d, e, f, g, h, i, j) { return [b, c, d, e, f, g, h, i || "." + d3_scale_linearFormatPrecision(j, range), j].join(""); })
-      : ",." + d3_scale_linearPrecision(range[2]) + "f");
+  if (format) {
+    var match = d3_format_re.exec(format);
+    match.shift();
+    if (match[8] === "s") {
+      var prefix = d3.formatPrefix(Math.max(abs(range[0]), abs(range[1])) * (match[7] ? Math.pow(10, -match[7].substring(1)) : 1));
+      match[7] = "." + d3_scale_linearPrecision(prefix.scale(range[2])), match[8] = "f";
+      format = d3.format(match.join(""));
+      return function(d) {
+        return format(prefix.scale(d)) + prefix.symbol;
+      };
+    }
+    if (!match[7]) match[7] = "." + d3_scale_linearFormatPrecision(match[8], range);
+    format = match.join("");
+  } else {
+    format = ",." + d3_scale_linearPrecision(range[2]) + "f";
+  }
+  return d3.format(format);
 }
 
 var d3_scale_linearFormatSignificant = {s: 1, g: 1, p: 1, r: 1, e: 1};

test/scale/linear-test.js

@@ -216,11 +216,23 @@ suite.addBatch({
     "tickFormat": {
       "applies automatic precision when not explicitly specified": function(d3) {
         var x = d3.scale.linear();
-        assert.strictEqual(x.tickFormat(10, "f")(Math.PI), "3.1")
+        assert.strictEqual(x.tickFormat(10, "f")(Math.PI), "3.1");
         assert.strictEqual(x.tickFormat(100, "f")(Math.PI), "3.14");
         assert.strictEqual(x.tickFormat(100, "$f")(Math.PI), "$3.14");
         assert.strictEqual(x.domain([0, 100]).tickFormat(100, "%")(Math.PI), "314%");
       },
+      "applies fixed-scale SI-prefix notation": function(d3) {
+        var x = d3.scale.linear().domain([0, 1e6]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, "s")), ["0.0M", "0.1M", "0.2M", "0.3M", "0.4M", "0.5M", "0.6M", "0.7M", "0.8M", "0.9M", "1.0M"]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, "+$s")), ["+$0.0M", "+$0.1M", "+$0.2M", "+$0.3M", "+$0.4M", "+$0.5M", "+$0.6M", "+$0.7M", "+$0.8M", "+$0.9M", "+$1.0M"]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, ".1s")), ["0k", "100k", "200k", "300k", "400k", "500k", "600k", "700k", "800k", "900k", "1000k"]);
+        var x = d3.scale.linear().domain([0, 1e5]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, "s")), ["0k", "10k", "20k", "30k", "40k", "50k", "60k", "70k", "80k", "90k", "100k"]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, ".-1s")), ["0.00M", "0.01M", "0.02M", "0.03M", "0.04M", "0.05M", "0.06M", "0.07M", "0.08M", "0.09M", "0.10M"]);
+        var x = d3.scale.linear().domain([0, 1e-4]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, "s")), ["0.00m", "0.01m", "0.02m", "0.03m", "0.04m", "0.05m", "0.06m", "0.07m", "0.08m", "0.09m", "0.10m"]);
+        assert.deepEqual(x.ticks(10).map(x.tickFormat(10, ".2s")), ["0µ", "10µ", "20µ", "30µ", "40µ", "50µ", "60µ", "70µ", "80µ", "90µ", "100µ"]);
+      },
       "if count is not specified, defaults to 10": function(d3) {
         var x = d3.scale.linear();
         assert.strictEqual(x.tickFormat()(Math.PI), "3.1");