Add d3.piecewise.

README.md

@@ -223,3 +223,13 @@ Returns a uniform nonrational B-spline interpolator through the specified array
 <a href="#interpolateBasisClosed" name="interpolateBasisClosed">#</a> d3.<b>interpolateBasisClosed</b>(<i>values</i>) [<>](https://github.com/d3/d3-interpolate/blob/master/src/basisClosed.js "Source")
 
 Returns a uniform nonrational B-spline interpolator through the specified array of *values*, which must be numbers. The control points are implicitly repeated such that the resulting one-dimensional spline has cyclical C² continuity when repeated around *t* in [0,1]. See also [d3.curveBasisClosed](https://github.com/d3/d3-shape#curveBasisClosed).
+
+### Piecewise
+
+<a name="piecewise" href="#piecewise">#</a> d3.<b>piecewise</b>(<i>interpolate</i>, <i>values</i>) [<>](https://github.com/d3/d3-interpolate/blob/master/src/piecewise.js "Source")
+
+Returns a piecewise interpolator, composing interpolators for each adjacent pair of *values*. The returned interpolator maps *t* in [0, 1 / (*n* - 1)] to *interpolate*(*values*[0], *values*[1]), *t* in [1 / (*n* - 1), 2 / (*n* - 1)] to *interpolate*(*values*[1], *values*[2]), and so on, where *n* = *values*.length. In effect, this is a lightweight [linear scale](https://github.com/d3/d3-scale/blob/master/README.md#linear-scales). For example, to blend through red, green and blue:
+
+```js
+var interpolate = d3.piecewise(d3.interpolateRgb.gamma(2.2), ["red", "green", "blue"]);
+```

index.js

@@ -14,4 +14,5 @@ export {default as interpolateHsl, hslLong as interpolateHslLong} from "./src/hs
 export {default as interpolateLab} from "./src/lab";
 export {default as interpolateHcl, hclLong as interpolateHclLong} from "./src/hcl";
 export {default as interpolateCubehelix, cubehelixLong as interpolateCubehelixLong} from "./src/cubehelix";
+export {default as piecewise} from "./src/piecewise";
 export {default as quantize} from "./src/quantize";

src/piecewise.js

@@ -0,0 +1,8 @@
+export default function piecewise(interpolate, values) {
+  var i = 0, n = values.length - 1, v = values[0], I = new Array(n < 0 ? 0 : n);
+  while (i < n) I[i] = interpolate(v, v = values[++i]);
+  return function(t) {
+    var i = Math.max(0, Math.min(n - 1, Math.floor(t *= n)));
+    return I[i](t - i);
+  };
+}