Index: node_modules/d3-force/src/y.js
===================================================================
--- node_modules/d3-force/src/y.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-force/src/y.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,41 @@
+import constant from "./constant.js";
+
+export default function(y) {
+  var strength = constant(0.1),
+      nodes,
+      strengths,
+      yz;
+
+  if (typeof y !== "function") y = constant(y == null ? 0 : +y);
+
+  function force(alpha) {
+    for (var i = 0, n = nodes.length, node; i < n; ++i) {
+      node = nodes[i], node.vy += (yz[i] - node.y) * strengths[i] * alpha;
+    }
+  }
+
+  function initialize() {
+    if (!nodes) return;
+    var i, n = nodes.length;
+    strengths = new Array(n);
+    yz = new Array(n);
+    for (i = 0; i < n; ++i) {
+      strengths[i] = isNaN(yz[i] = +y(nodes[i], i, nodes)) ? 0 : +strength(nodes[i], i, nodes);
+    }
+  }
+
+  force.initialize = function(_) {
+    nodes = _;
+    initialize();
+  };
+
+  force.strength = function(_) {
+    return arguments.length ? (strength = typeof _ === "function" ? _ : constant(+_), initialize(), force) : strength;
+  };
+
+  force.y = function(_) {
+    return arguments.length ? (y = typeof _ === "function" ? _ : constant(+_), initialize(), force) : y;
+  };
+
+  return force;
+}