fix axis range animations

... by skipping Cartesian.plot step when no trace data need to
    be transitioned.

Author: etpinard

src/plots/cartesian/index.js

@@ -122,15 +122,32 @@ exports.finalizeSubplots = function(layoutIn, layoutOut) {
     }
 };
 
+/**
+ * Cartesian.plot
+ *
+ * @param {DOM div | object} gd
+ * @param {array | null} (optional) traces
+ *  array of traces indices to plot
+ *  if undefined, plots all cartesian traces,
+ *  if null, plots no traces
+ * @param {object} (optional) transitionOpts
+ *  transition option object
+ * @param {function} (optional) makeOnCompleteCallback
+ *  transition make callback function from Plots.transition
+ */
 exports.plot = function(gd, traces, transitionOpts, makeOnCompleteCallback) {
     var fullLayout = gd._fullLayout;
     var subplots = fullLayout._subplots.cartesian;
     var calcdata = gd.calcdata;
     var i;
 
-    // If traces is not provided, then it's a complete replot and missing
-    // traces are removed
-    if(!Array.isArray(traces)) {
+    if(traces === null) {
+        // this means no updates required, must return here
+        // so that plotOne doesn't remove the trace layers
+        return;
+    } else if(!Array.isArray(traces)) {
+        // If traces is not provided, then it's a complete replot and missing
+        // traces are removed
         traces = [];
         for(i = 0; i < calcdata.length; i++) traces.push(i);
     }

src/plots/plots.js

@@ -2333,6 +2333,9 @@ plots.transition = function(gd, data, layout, traces, frameOpts, transitionOpts)
             if(hasAxisTransition) {
                 traceTransitionOpts = Lib.extendFlat({}, transitionOpts);
                 traceTransitionOpts.duration = 0;
+                // This means do not transition traces,
+                // this happens on layout-only (e.g. axis range) animations
+                transitionedTraces = null;
             } else {
                 traceTransitionOpts = transitionOpts;
             }

test/jasmine/tests/animate_test.js

@@ -1,5 +1,6 @@
 var Plotly = require('@lib/index');
 var Lib = require('@src/lib');
+var Registry = require('@src/registry');
 var Plots = Plotly.Plots;
 
 var createGraphDiv = require('../assets/create_graph_div');
@@ -836,4 +837,43 @@ describe('animating scatter traces', function() {
             expect(gd.calcdata[0][0].y).toEqual(3);
         }).catch(failTest).then(done);
     });
+
+    it('should animate axis ranges using the less number of steps', function(done) {
+        Plotly.plot(gd, [{
+            y: [1, 2, 1]
+        }, {
+            type: 'bar',
+            y: [2, 1, 2]
+        }])
+        .then(function() {
+            spyOn(gd._fullData[0]._module.basePlotModule, 'transitionAxes').and.callThrough();
+            spyOn(gd._fullData[0]._module, 'plot').and.callThrough();
+            spyOn(gd._fullData[1]._module, 'plot').and.callThrough();
+            spyOn(Registry, 'call').and.callThrough();
+
+            return Plotly.animate('graph', {
+                layout: {
+                    xaxis: {range: [0.45, 0.55]},
+                    yaxis: {range: [0.45, 0.55]}
+                }
+            }, {
+                transition: {duration: 500},
+                frame: {redraw: false}
+            });
+        })
+        .then(function() {
+            // the only redraw should occur during Cartesian.transitionAxes,
+            // where Registry.call('relayout') is called leading to a _module.plot call
+            expect(gd._fullData[0]._module.basePlotModule.transitionAxes).toHaveBeenCalledTimes(1);
+            expect(gd._fullData[0]._module.plot).toHaveBeenCalledTimes(1);
+            expect(gd._fullData[1]._module.plot).toHaveBeenCalledTimes(1);
+            expect(Registry.call).toHaveBeenCalledTimes(1);
+
+            var calls = Registry.call.calls.all();
+            expect(calls.length).toBe(1, 'just one Registry.call call');
+            expect(calls[0].args[0]).toBe('relayout', 'called Registry.call with');
+        })
+        .catch(failTest)
+        .then(done);
+    });
 });