Index: node_modules/d3-transition/src/transition/on.js
===================================================================
--- node_modules/d3-transition/src/transition/on.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-transition/src/transition/on.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,32 @@
+import {get, set, init} from "./schedule.js";
+
+function start(name) {
+  return (name + "").trim().split(/^|\s+/).every(function(t) {
+    var i = t.indexOf(".");
+    if (i >= 0) t = t.slice(0, i);
+    return !t || t === "start";
+  });
+}
+
+function onFunction(id, name, listener) {
+  var on0, on1, sit = start(name) ? init : set;
+  return function() {
+    var schedule = sit(this, id),
+        on = schedule.on;
+
+    // If this node shared a dispatch with the previous node,
+    // just assign the updated shared dispatch and we’re done!
+    // Otherwise, copy-on-write.
+    if (on !== on0) (on1 = (on0 = on).copy()).on(name, listener);
+
+    schedule.on = on1;
+  };
+}
+
+export default function(name, listener) {
+  var id = this._id;
+
+  return arguments.length < 2
+      ? get(this.node(), id).on.on(name)
+      : this.each(onFunction(id, name, listener));
+}