Index: node_modules/d3-polygon/src/length.js
===================================================================
--- node_modules/d3-polygon/src/length.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-polygon/src/length.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,23 @@
+export default function(polygon) {
+  var i = -1,
+      n = polygon.length,
+      b = polygon[n - 1],
+      xa,
+      ya,
+      xb = b[0],
+      yb = b[1],
+      perimeter = 0;
+
+  while (++i < n) {
+    xa = xb;
+    ya = yb;
+    b = polygon[i];
+    xb = b[0];
+    yb = b[1];
+    xa -= xb;
+    ya -= yb;
+    perimeter += Math.hypot(xa, ya);
+  }
+
+  return perimeter;
+}