Coerce scale output to numbers.

Fixes d3/d3-scale#107.

src/axis.js

@@ -15,11 +15,17 @@ function translateY(y) {
   return "translate(0," + (y + 0.5) + ")";
 }
 
+function number(scale) {
+  return function(d) {
+    return +scale(d);
+  };
+}
+
 function center(scale) {
   var offset = Math.max(0, scale.bandwidth() - 1) / 2; // Adjust for 0.5px offset.
   if (scale.round()) offset = Math.round(offset);
   return function(d) {
-    return scale(d) + offset;
+    return +scale(d) + offset;
   };
 }
 
@@ -43,9 +49,9 @@ function axis(orient, scale) {
         format = tickFormat == null ? (scale.tickFormat ? scale.tickFormat.apply(scale, tickArguments) : identity) : tickFormat,
         spacing = Math.max(tickSizeInner, 0) + tickPadding,
         range = scale.range(),
-        range0 = range[0] + 0.5,
-        range1 = range[range.length - 1] + 0.5,
-        position = (scale.bandwidth ? center : identity)(scale.copy()),
+        range0 = +range[0] + 0.5,
+        range1 = +range[range.length - 1] + 0.5,
+        position = (scale.bandwidth ? center : number)(scale.copy()),
         selection = context.selection ? context.selection() : context,
         path = selection.selectAll(".domain").data([null]),
         tick = selection.selectAll(".tick").data(values, scale).order(),