Fix a crash when rounding very small numbers.

d3.js

@@ -525,7 +525,7 @@ var d3_format_types = {
   f: function(x, p) { return x.toFixed(p); },
   r: function(x, p) {
     var n = 1 + Math.floor(1e-15 + Math.log(x) / Math.LN10);
-    return d3.round(x, p - n).toFixed(Math.max(0, p - n));
+    return d3.round(x, p - n).toFixed(Math.max(0, Math.min(20, p - n)));
   }
 };
 

src/core/format.js

@@ -67,7 +67,7 @@ var d3_format_types = {
   f: function(x, p) { return x.toFixed(p); },
   r: function(x, p) {
     var n = 1 + Math.floor(1e-15 + Math.log(x) / Math.LN10);
-    return d3.round(x, p - n).toFixed(Math.max(0, p - n));
+    return d3.round(x, p - n).toFixed(Math.max(0, Math.min(20, p - n)));
   }
 };
 

test/core/format-test.js

@@ -105,6 +105,10 @@ suite.addBatch({
       assert.strictEqual(format(".5r")(123.45), "123.45");
       assert.strictEqual(format(".6r")(123.45), "123.450");
     },
+    "can round very small numbers": function(format) {
+      var f = format(".2r");
+      assert.strictEqual(f(1e-22), "0.00000000000000000000");
+    },
     "can group thousands": function(format) {
       var f = format(",d");
       assert.strictEqual(f(0), "0");