From f61187fc6c00e3df6217ca19a1ecabc6ef1b4241 Mon Sep 17 00:00:00 2001
From: wbrgss <williamb@plot.ly>
Date: Mon, 14 Dec 2020 18:26:39 -0500
Subject: [PATCH 1/2] Only run emptyCategories loop when restyle or relayout
 calc flag is true

---
 src/plot_api/plot_api.js | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/src/plot_api/plot_api.js b/src/plot_api/plot_api.js
index ffd6bb863b5..941f172e269 100644
--- a/src/plot_api/plot_api.js
+++ b/src/plot_api/plot_api.js
@@ -2737,16 +2737,6 @@ function react(gd, data, layout, config) {
 
         applyUIRevisions(gd.data, gd.layout, oldFullData, oldFullLayout);
 
-        var allNames = Object.getOwnPropertyNames(oldFullLayout);
-        for(var q = 0; q < allNames.length; q++) {
-            var name = allNames[q];
-            var start = name.substring(0, 5);
-            if(start === 'xaxis' || start === 'yaxis') {
-                var emptyCategories = oldFullLayout[name]._emptyCategories;
-                if(emptyCategories) emptyCategories();
-            }
-        }
-
         // "true" skips updating calcdata and remapping arrays from calcTransforms,
         // which supplyDefaults usually does at the end, but we may need to NOT do
         // if the diff (which we haven't determined yet) says we'll recalc
@@ -2772,10 +2762,22 @@ function react(gd, data, layout, config) {
 
         if(updateAutosize(gd)) relayoutFlags.layoutReplot = true;
 
-        // clear calcdata if required
-        if(restyleFlags.calc || relayoutFlags.calc) gd.calcdata = undefined;
+        // clear calcdata and empty categories if required
+        if(restyleFlags.calc || relayoutFlags.calc) {
+            gd.calcdata = undefined;
+            var allNames = Object.getOwnPropertyNames(newFullLayout);
+            for(var q = 0; q < allNames.length; q++) {
+                var name = allNames[q];
+                var start = name.substring(0, 5);
+                if(start === 'xaxis' || start === 'yaxis') {
+                    var emptyCategories = newFullLayout[name]._emptyCategories;
+                    if(emptyCategories) emptyCategories();
+                }
+            }
         // otherwise do the calcdata updates and calcTransform array remaps that we skipped earlier
-        else Plots.supplyDefaultsUpdateCalc(gd.calcdata, newFullData);
+        } else {
+            Plots.supplyDefaultsUpdateCalc(gd.calcdata, newFullData);
+        }
 
         // Note: what restyle/relayout use impliedEdits and clearAxisTypes for
         // must be handled by the user when using Plotly.react.

From eed5c8a2bceb6f6bb0a8c360bab26ea9cd8fd2df Mon Sep 17 00:00:00 2001
From: wbrgss <williamb@plot.ly>
Date: Tue, 15 Dec 2020 14:00:20 -0500
Subject: [PATCH 2/2] Add test for categories/axes label preservation on
 Plotly.react(gd, ...)

---
 test/jasmine/tests/axes_test.js | 58 +++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/test/jasmine/tests/axes_test.js b/test/jasmine/tests/axes_test.js
index 94d7979cce8..a3da155704f 100644
--- a/test/jasmine/tests/axes_test.js
+++ b/test/jasmine/tests/axes_test.js
@@ -6896,6 +6896,64 @@ describe('more react tests', function() {
     });
 });
 
+describe('category preservation tests on gd passed to Plotly.react()', function() {
+    var gd;
+
+    beforeEach(function() {
+        gd = createGraphDiv();
+    });
+
+    afterEach(destroyGraphDiv);
+
+    function _hover(gd, opts) {
+        Fx.hover(gd, opts);
+        // needed for successive hover events
+        Lib.clearThrottle();
+    }
+
+    it('should preserve categories and axis ticklabels', function(done) {
+        var fig = {
+            data: [{
+                type: 'bar',
+                y: [3, 5, 3, 2],
+                x: ['a', 'b', 'c', 'd']
+            }],
+            layout: {
+                width: 500,
+                height: 500
+            }
+        };
+
+        Plotly.newPlot(gd, fig)
+        .then(function(gd) {
+            return Plotly.react(gd, fig);
+        })
+        .then(function() {
+            expect(gd._fullLayout.xaxis._categories).toEqual(['a', 'b', 'c', 'd']);
+            expect(gd._fullLayout.xaxis._categoriesMap).toEqual({a: 0, b: 1, c: 2, d: 3});
+        })
+        .then(function() {
+            _hover(gd, { xval: fig.data[0].x.indexOf('a') });
+            expect(d3.selectAll('g.axistext').select('text').html()).toEqual('a');
+        })
+        .then(function() {
+            _hover(gd, { xval: fig.data[0].x.indexOf('b') });
+            expect(d3.selectAll('g.axistext').select('text').html()).toEqual('b');
+        })
+        .then(function() {
+            _hover(gd, { xval: fig.data[0].x.indexOf('c') });
+            expect(d3.selectAll('g.axistext').select('text').html()).toEqual('c');
+        })
+        .then(function() {
+            _hover(gd, { xval: fig.data[0].x.indexOf('d') });
+            expect(d3.selectAll('g.axistext').select('text').html()).toEqual('d');
+        })
+
+        .catch(failTest)
+        .then(done);
+    });
+});
+
 describe('more matching axes tests', function() {
     var gd;