Index: node_modules/d3-array/src/sum.js
===================================================================
--- node_modules/d3-array/src/sum.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-array/src/sum.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,18 @@
+export default function sum(values, valueof) {
+  let sum = 0;
+  if (valueof === undefined) {
+    for (let value of values) {
+      if (value = +value) {
+        sum += value;
+      }
+    }
+  } else {
+    let index = -1;
+    for (let value of values) {
+      if (value = +valueof(value, ++index, values)) {
+        sum += value;
+      }
+    }
+  }
+  return sum;
+}