Add d3.forceBox.

src/box.js

@@ -0,0 +1,27 @@
+export default function(extent) {
+  var nodes,
+      x0 = +extent[0][0],
+      y0 = +extent[0][1],
+      x1 = +extent[1][0],
+      y1 = +extent[1][1];
+
+  function force() {
+    for (var i = 0, n = nodes.length, node; i < n; ++i) {
+      node = nodes[i];
+      if (node.x < x0) node.x = x0;
+      else if (node.x > x1) node.x = x1;
+      if (node.y < y0) node.y = y0;
+      else if (node.y > y1) node.y = y1;
+    }
+  }
+
+  force.initialize = function(_) {
+    nodes = _;
+  };
+
+  force.extent = function(_) {
+    return arguments.length ? (x0 = +_[0][0], y0 = +_[0][1], x1 = +_[1][0], y1 = +_[1][1], force) : [[x0, y0], [x1, y1]];
+  };
+
+  return force;
+}