feat: add new file

frontend/node_modules/d3-array/src/mean.js

@@ -0,0 +1,19 @@
+export default function mean(values, valueof) {
+  let count = 0;
+  let sum = 0;
+  if (valueof === undefined) {
+    for (let value of values) {
+      if (value != null && (value = +value) >= value) {
+        ++count, sum += value;
+      }
+    }
+  } else {
+    let index = -1;
+    for (let value of values) {
+      if ((value = valueof(value, ++index, values)) != null && (value = +value) >= value) {
+        ++count, sum += value;
+      }
+    }
+  }
+  if (count) return sum / count;
+}