better ordering of trace hoverlabels for matching positions

src/components/fx/hover.js

@@ -951,21 +951,32 @@ function createHoverText(hoverData, opts, gd) {
 function hoverAvoidOverlaps(hoverData, ax, fullLayout) {
     var nummoves = 0;
 
-        // make groups of touching points
+    var axSign = 1;
+
+    // make groups of touching points
     var pointgroups = hoverData.map(function(d, i) {
         var axis = d[ax];
+        var axIsX = axis._id.charAt(0) === 'x';
+        var rng = axis.range;
+        if(!i && rng && ((rng[0] > rng[1]) !== axIsX)) axSign = -1;
         return [{
             i: i,
+            traceIndex: d.trace.index,
             dp: 0,
             pos: d.pos,
             posref: d.posref,
-            size: d.by * (axis._id.charAt(0) === 'x' ? YFACTOR : 1) / 2,
+            size: d.by * (axIsX ? YFACTOR : 1) / 2,
             pmin: 0,
-            pmax: (axis._id.charAt(0) === 'x' ? fullLayout.width : fullLayout.height)
+            pmax: (axIsX ? fullLayout.width : fullLayout.height)
         }];
     })
     .sort(function(a, b) {
-        return a[0].posref - b[0].posref;
+        return (a[0].posref - b[0].posref) ||
+            // for equal positions, sort trace indices increasing or decreasing
+            // depending on whether the axis is reversed or not... so stacked
+            // traces will generally keep their order even if one trace adds
+            // nothing to the stack.
+            (axSign * (b[0].traceIndex - a[0].traceIndex));
     });
 
     var donepositioning, topOverlap, bottomOverlap, i, j, pti, sumdp;

test/jasmine/assets/custom_assertions.js

@@ -122,6 +122,7 @@ exports.assertHoverLabelContent = function(expectation, msg) {
         expect(ptCnt)
             .toBe(expectation.name.length, ptMsg + ' # of visible labels');
 
+        var bboxes = [];
         d3.selectAll(ptSelector).each(function(_, i) {
             assertLabelContent(
                 d3.select(this).select('text.nums'),
@@ -133,7 +134,20 @@ exports.assertHoverLabelContent = function(expectation, msg) {
                 expectation.name[i],
                 ptMsg + ' (name ' + i + ')'
             );
+            bboxes.push({bbox: this.getBoundingClientRect(), index: i});
         });
+        if(expectation.vOrder) {
+            bboxes.sort(function(a, b) {
+                return (a.bbox.top + a.bbox.bottom - b.bbox.top - b.bbox.bottom) / 2;
+            });
+            expect(bboxes.map(function(d) { return d.index; })).toEqual(expectation.vOrder);
+        }
+        if(expectation.hOrder) {
+            bboxes.sort(function(a, b) {
+                return (b.bbox.left + b.bbox.right - a.bbox.left - a.bbox.right) / 2;
+            });
+            expect(bboxes.map(function(d) { return d.index; })).toEqual(expectation.hOrder);
+        }
     } else {
         if(expectation.nums) {
             fail(ptMsg + ': expecting *nums* labels, did not find any.');

test/jasmine/tests/hover_label_test.js

@@ -520,7 +520,92 @@ describe('hover info', function() {
         Lib.clearThrottle();
     }
 
-    describe('\'hover info for x/y/z traces', function() {
+    describe('hover label order for stacked traces with zeros', function() {
+        var gd;
+        beforeEach(function() {
+            gd = createGraphDiv();
+        });
+
+        it('puts the top trace on top', function(done) {
+            Plotly.plot(gd, [
+                {y: [1, 2, 3], type: 'bar', name: 'a'},
+                {y: [2, 0, 1], type: 'bar', name: 'b'},
+                {y: [1, 0, 1], type: 'bar', name: 'c'},
+                {y: [2, 1, 0], type: 'bar', name: 'd'}
+            ], {
+                width: 500,
+                height: 400,
+                margin: {l: 0, t: 0, r: 0, b: 0},
+                barmode: 'stack'
+            })
+            .then(function() {
+                _hover(gd, 250, 250);
+                assertHoverLabelContent({
+                    nums: ['2', '0', '0', '1'],
+                    name: ['a', 'b', 'c', 'd'],
+                    // a, b, c are all in the same place but keep their order
+                    // d is included mostly as a sanity check
+                    vOrder: [3, 2, 1, 0],
+                    axis: '1'
+                });
+
+                // reverse the axis, labels should reverse
+                return Plotly.relayout(gd, 'yaxis.range', gd.layout.yaxis.range.slice().reverse());
+            })
+            .then(function() {
+                _hover(gd, 250, 250);
+                assertHoverLabelContent({
+                    nums: ['2', '0', '0', '1'],
+                    name: ['a', 'b', 'c', 'd'],
+                    vOrder: [0, 1, 2, 3],
+                    axis: '1'
+                });
+            })
+            .catch(failTest)
+            .then(done);
+        });
+
+        it('puts the right trace on the right', function(done) {
+            Plotly.plot(gd, [
+                {x: [1, 2, 3], type: 'bar', name: 'a', orientation: 'h'},
+                {x: [2, 0, 1], type: 'bar', name: 'b', orientation: 'h'},
+                {x: [1, 0, 1], type: 'bar', name: 'c', orientation: 'h'},
+                {x: [2, 1, 0], type: 'bar', name: 'd', orientation: 'h'}
+            ], {
+                width: 500,
+                height: 400,
+                margin: {l: 0, t: 0, r: 0, b: 0},
+                barmode: 'stack'
+            })
+            .then(function() {
+                _hover(gd, 250, 250);
+                assertHoverLabelContent({
+                    nums: ['2', '0', '0', '1'],
+                    name: ['a', 'b', 'c', 'd'],
+                    // a, b, c are all in the same place but keep their order
+                    // d is included mostly as a sanity check
+                    hOrder: [3, 2, 1, 0],
+                    axis: '1'
+                });
+
+                // reverse the axis, labels should reverse
+                return Plotly.relayout(gd, 'xaxis.range', gd.layout.xaxis.range.slice().reverse());
+            })
+            .then(function() {
+                _hover(gd, 250, 250);
+                assertHoverLabelContent({
+                    nums: ['2', '0', '0', '1'],
+                    name: ['a', 'b', 'c', 'd'],
+                    hOrder: [0, 1, 2, 3],
+                    axis: '1'
+                });
+            })
+            .catch(failTest)
+            .then(done);
+        });
+    });
+
+    describe('hover info for x/y/z traces', function() {
         var gd;
         beforeEach(function() {
             gd = createGraphDiv();