Defer persistence until transition start.

The zoom behavior was immediately persisting the new view when the transition
was scheduled, rather than waiting until the transition started. By waiting
until the transition starts, it’s possible to schedule chained transitions to
different views.

d3.js

@@ -1185,15 +1185,14 @@ d3 = function() {
     }
     zoom.event = function(g) {
       g.each(function() {
-        var event_ = event.of(this, arguments), view1 = view, view0 = this.__chart__ || {
+        var event_ = event.of(this, arguments), view1 = view;
+        if (d3_transitionInheritId) {
+          d3.select(this).transition().each("start.zoom", function() {
+            view = this.__chart__ || {
               x: 0,
               y: 0,
               k: 1
             };
-        this.__chart__ = view;
-        if (d3_transitionInheritId) {
-          d3.select(this).transition().each("start.zoom", function() {
-            view = view0;
             zoomstarted(event_);
           }).tween("zoom:zoom", function() {
             var dx = size[0], dy = size[1], cx = dx / 2, cy = dy / 2, i = d3.interpolateZoom([ (cx - view.x) / view.k, (cy - view.y) / view.k, dx / view.k ], [ (cx - view1.x) / view1.k, (cy - view1.y) / view1.k, dx / view1.k ]);
@@ -1210,6 +1209,7 @@ d3 = function() {
             zoomended(event_);
           });
         } else {
+          this.__chart__ = view;
           zoomstarted(event_);
           zoomed(event_);
           zoomended(event_);

src/behavior/zoom.js

@@ -39,13 +39,11 @@ d3.behavior.zoom = function() {
   zoom.event = function(g) {
     g.each(function() {
       var event_ = event.of(this, arguments),
-          view1 = view,
-          view0 = this.__chart__ || {x: 0, y: 0, k: 1};
-      this.__chart__ = view;
+          view1 = view;
       if (d3_transitionInheritId) {
           d3.select(this).transition()
               .each("start.zoom", function() {
-                view = view0; // pre-transition state
+                view = this.__chart__ || {x: 0, y: 0, k: 1}; // pre-transition state
                 zoomstarted(event_);
               })
               .tween("zoom:zoom", function() {
@@ -67,6 +65,7 @@ d3.behavior.zoom = function() {
                 zoomended(event_);
               });
       } else {
+        this.__chart__ = view;
         zoomstarted(event_);
         zoomed(event_);
         zoomended(event_);