bar: add object constancy to transitions

Author: antoinerg

src/traces/bar/plot.js

@@ -31,6 +31,13 @@ var appendArrayPointValue = require('../../components/fx/helpers').appendArrayPo
 // padding in pixels around text
 var TEXTPAD = 3;
 
+function keyFunc(d) {return d.id;}
+function getKeyFunc(trace) {
+    if(trace.ids) {
+        return keyFunc;
+    }
+}
+
 function dirSign(a, b) {
     return (a < b) ? 1 : -1;
 }
@@ -103,7 +110,8 @@ function plot(gd, plotinfo, cdModule, traceLayer, opts, makeOnCompleteCallback)
 
         var pointGroup = Lib.ensureSingle(plotGroup, 'g', 'points');
 
-        var bars = pointGroup.selectAll('g.point').data(Lib.identity);
+        var keyFunc = getKeyFunc(trace);
+        var bars = pointGroup.selectAll('g.point').data(Lib.identity, keyFunc);
 
         bars.enter().append('g')
             .classed('point', true);

test/image/mocks/animation_bar.json

@@ -3,6 +3,7 @@
     "type": "bar",
     "x": ["A", "B", "C"],
     "y": [24, 5, 8],
+    "ids": ["A", "B", "C"],
     "error_y": {"array": [3, 2, 1]}
     }],
   "layout": {
@@ -17,6 +18,7 @@
       {"data": [{"width": 0.25}]},
       {"data": [{"marker": {"line": {"width": 10}}}]},
       {"data": [{"marker": {"line": {"color": ["orange", "yellow", "blue"]}}}]},
+      {"data": [{"ids": ["B", "C", "A"]}]},
       {"layout": {"yaxis": {"range": [0, 20]}}}
     ]
 }