Index: node_modules/d3-force/src/center.js
===================================================================
--- node_modules/d3-force/src/center.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-force/src/center.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,40 @@
+export default function(x, y) {
+  var nodes, strength = 1;
+
+  if (x == null) x = 0;
+  if (y == null) y = 0;
+
+  function force() {
+    var i,
+        n = nodes.length,
+        node,
+        sx = 0,
+        sy = 0;
+
+    for (i = 0; i < n; ++i) {
+      node = nodes[i], sx += node.x, sy += node.y;
+    }
+
+    for (sx = (sx / n - x) * strength, sy = (sy / n - y) * strength, i = 0; i < n; ++i) {
+      node = nodes[i], node.x -= sx, node.y -= sy;
+    }
+  }
+
+  force.initialize = function(_) {
+    nodes = _;
+  };
+
+  force.x = function(_) {
+    return arguments.length ? (x = +_, force) : x;
+  };
+
+  force.y = function(_) {
+    return arguments.length ? (y = +_, force) : y;
+  };
+
+  force.strength = function(_) {
+    return arguments.length ? (strength = +_, force) : strength;
+  };
+
+  return force;
+}