Plot.movingAverage

Author: mbostock

src/index.js

@@ -16,4 +16,5 @@ export {Text, text, textX, textY} from "./marks/text.js";
 export {TickX, TickY, tickX, tickY} from "./marks/tick.js";
 export {bin1, bin2} from "./transforms/bin.js";
 export {group1, group2} from "./transforms/group.js";
+export {movingAverage} from "./transforms/movingAverage.js";
 export {stackX, stackY} from "./transforms/stack.js";

src/mark.js

@@ -83,16 +83,18 @@ function Channel(data, {scale, type, value}) {
   return {
     scale,
     type,
-    value: typeof value === "function" ? Array.from(data, value) : arrayify(value),
+    value: valueof(data, value),
     label: value ? value.label : undefined
   };
 }
 
 // This allows transforms to behave equivalently to channels.
-export function valueof(data, value) {
-  return typeof value === "string" ? Array.from(data, field(value))
-    : typeof value === "function" ? Array.from(data, value)
-    : arrayify(value);
+export function valueof(data, value, type) {
+  const array = type === undefined ? Array : type;
+  return typeof value === "string" ? array.from(data, field(value))
+    : typeof value === "function" ? array.from(data, value)
+    : value && typeof value.transform === "function" ? arrayify(value.transform(data), type)
+    : arrayify(value, type); // preserve undefined type
 }
 
 export const field = label => Object.assign(d => d[label], {label});

src/transforms/movingAverage.js

@@ -0,0 +1,29 @@
+import {field, identity, valueof} from "../mark.js";
+
+// TODO allow partially-defined data
+export function movingAverage(N, value = identity) {
+  if (!((N = Math.floor(N)) > 0)) throw new Error("invalid N");
+  if (typeof value === "string") value = field(value);
+  else if (typeof value !== "function") throw new Error("invalid value");
+  return {
+    transform(data) {
+      let i = 0;
+      let sum = 0;
+      const values = valueof(data, value, Float64Array);
+      const means = new Float64Array(values.length);
+      for (let n = Math.min(N - 1, values.length); i < n; ++i) {
+        means[i] = NaN;
+        sum += values[i];
+      }
+      for (let n = values.length; i < n; ++i) {
+        sum += values[i];
+        means[i] = sum / N;
+        sum -= values[i - N + 1];
+      }
+      means.subarray(0, N >> 1).reverse();
+      means.subarray(N >> 1).reverse();
+      means.reverse();
+      return means;
+    }
+  };
+}