Merge branch 'zoom-center' into 3.3

Conflicts:
	src/behavior/zoom.js

d3.js

@@ -1138,7 +1138,7 @@ d3 = function() {
     return d3.rebind(drag, event, "on");
   };
   d3.behavior.zoom = function() {
-    var translate = [ 0, 0 ], translate0, scale = 1, scaleExtent = d3_behavior_zoomInfinity, mousedown = "mousedown.zoom", mousemove = "mousemove.zoom", mouseup = "mouseup.zoom", mousewheelTimer, touchstart = "touchstart.zoom", touchmove = "touchmove.zoom", touchend = "touchend.zoom", touchtime, event = d3_eventDispatch(zoom, "zoomstart", "zoom", "zoomend"), x0, x1, y0, y1;
+    var translate = [ 0, 0 ], translate0, center, scale = 1, scaleExtent = d3_behavior_zoomInfinity, mousedown = "mousedown.zoom", mousemove = "mousemove.zoom", mouseup = "mouseup.zoom", mousewheelTimer, touchstart = "touchstart.zoom", touchmove = "touchmove.zoom", touchend = "touchend.zoom", touchtime, event = d3_eventDispatch(zoom, "zoomstart", "zoom", "zoomend"), x0, x1, y0, y1;
     function zoom() {
       this.on(mousedown, mousedowned).on(d3_behavior_zoomWheel + ".zoom", mousewheeled).on(mousemove, mousewheelreset).on("dblclick.zoom", dblclicked).on(touchstart, touchstarted);
     }
@@ -1159,6 +1159,11 @@ d3 = function() {
       scaleExtent = x == null ? d3_behavior_zoomInfinity : x.map(Number);
       return zoom;
     };
+    zoom.center = function(_) {
+      if (!arguments.length) return center;
+      center = _ && _.map(Number);
+      return zoom;
+    };
     zoom.x = function(z) {
       if (!arguments.length) return x1;
       x1 = z;
@@ -1293,9 +1298,10 @@ d3 = function() {
         zoomended(event_);
       }, 50);
       d3_eventPreventDefault();
-      if (!translate0) translate0 = location(d3.mouse(this));
+      var point = center || d3.mouse(this);
+      if (!translate0) translate0 = location(point);
       scaleTo(Math.pow(2, d3_behavior_zoomDelta() * .002) * scale);
-      translateTo(d3.mouse(this), translate0);
+      translateTo(point, translate0);
       zoomed(event_);
     }
     function mousewheelreset() {

src/behavior/zoom.js

@@ -10,6 +10,7 @@ import "behavior";
 d3.behavior.zoom = function() {
   var translate = [0, 0],
       translate0, // translate when we started zooming (to avoid drift)
+      center, // desired position of translate0 after zooming
       scale = 1,
       scaleExtent = d3_behavior_zoomInfinity,
       mousedown = "mousedown.zoom",
@@ -54,6 +55,12 @@ d3.behavior.zoom = function() {
     return zoom;
   };
 
+  zoom.center = function(_) {
+    if (!arguments.length) return center;
+    center = _ && _.map(Number);
+    return zoom;
+  };
+
   zoom.x = function(z) {
     if (!arguments.length) return x1;
     x1 = z;
@@ -215,9 +222,10 @@ d3.behavior.zoom = function() {
     else zoomstarted(event_);
     mousewheelTimer = setTimeout(function() { mousewheelTimer = null; zoomended(event_); }, 50);
     d3_eventPreventDefault();
-    if (!translate0) translate0 = location(d3.mouse(this));
+    var point = center || d3.mouse(this);
+    if (!translate0) translate0 = location(point);
     scaleTo(Math.pow(2, d3_behavior_zoomDelta() * .002) * scale);
-    translateTo(d3.mouse(this), translate0);
+    translateTo(point, translate0);
     zoomed(event_);
   }