feat: allow fixing the vertical order of Sankey nodes

src/traces/sankey/attributes.js

@@ -170,6 +170,16 @@ var attrs = (module.exports = overrideAll(
                 dflt: 'justify',
                 description: 'Sets the alignment method used to position the nodes along the horizontal axis.'
             },
+            sort: {
+                valType: 'enumerated',
+                values: ['auto', 'input'],
+                dflt: 'auto',
+                description: [
+                    'If the value is `auto` (the default), the vertical order of nodes will be determined automatically',
+                    'by the layout.',
+                    'If the value is `input`, the vertical order is kept the same as the order in the input node array'
+                ].join(' ')
+            },
             description: 'The nodes of the Sankey plot.'
         },
 

src/traces/sankey/defaults.js

@@ -34,6 +34,7 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
     handleHoverLabelDefaults(nodeIn, nodeOut, coerceNode, hoverlabelDefault);
     coerceNode('hovertemplate');
     coerceNode('align');
+    coerceNode('sort');
 
     var colors = layout.colorway;
 

src/traces/sankey/render.js

@@ -40,6 +40,7 @@ function sankeyModel(layout, d, traceIndex) {
         right: d3Sankey.sankeyRight,
         center: d3Sankey.sankeyCenter
     }[trace.node.align];
+    var input_sort = trace.node.sort === 'input';
 
     var width = layout.width * (domain.x[1] - domain.x[0]);
     var height = layout.height * (domain.y[1] - domain.y[0]);
@@ -48,6 +49,10 @@ function sankeyModel(layout, d, traceIndex) {
     var links = calcData._links;
     var circular = calcData.circular;
 
+    if(circular && input_sort) {
+        Lib.error('Circular Sankey diagrams do not support the "input" node.sort mode; falling back to the default sort.');
+    }
+
     // Select Sankey generator
     var sankey;
     if(circular) {
@@ -70,6 +75,11 @@ function sankeyModel(layout, d, traceIndex) {
       .nodes(nodes)
       .links(links);
 
+    // d3-sankey-circular does not support the nodeSort method
+    if(!circular) {
+        sankey.nodeSort(input_sort ? null : undefined);
+    }
+
     var graph = sankey();
 
     if(sankey.nodePadding() < nodePad) {

test/image/mocks/sankey_auto_sort.json

@@ -0,0 +1,22 @@
+{
+  "data": [
+    {
+      "type": "sankey",
+      "node": {
+        "label": ["0", "1", "2", "3", "4", "5"],
+        "align": "center",
+        "sort": "auto"
+      },
+      "link": {
+        "source": [0, 0, 0, 0, 1, 4],
+        "target": [1, 2, 3, 4, 5, 5],
+        "value": [1, 2, 1, 1, 1, 1]
+      }
+    }
+  ],
+  "layout": {
+    "title": { "text": "Sankey with automatic vertical node ordering" },
+    "width": 800,
+    "height": 800
+  }
+}

test/image/mocks/sankey_input_sort.json

@@ -0,0 +1,22 @@
+{
+  "data": [
+    {
+      "type": "sankey",
+      "node": {
+        "label": ["0", "1", "2", "3", "4", "5"],
+        "align": "center",
+        "sort": "input"
+      },
+      "link": {
+        "source": [0, 0, 0, 0, 1, 4],
+        "target": [1, 2, 3, 4, 5, 5],
+        "value": [1, 2, 1, 1, 1, 1]
+      }
+    }
+  ],
+  "layout": {
+    "title": { "text": "Sankey with fixed vertical node ordering" },
+    "width": 800,
+    "height": 800
+  }
+}

test/jasmine/tests/sankey_test.js

@@ -501,6 +501,27 @@ describe('sankey tests', function () {
                 .then(done, done.fail);
         });
 
+        it('falls back to the default sort for circular Sankey with node.sort set', function (done) {
+            var errors = [];
+            spyOn(Lib, 'error').and.callFake(function (msg) {
+                errors.push(msg);
+            });
+
+            var mockCircularCopy = Lib.extendDeep({}, mockCircular);
+            mockCircularCopy.data[0].node.sort = 'input';
+
+            Plotly.newPlot(gd, mockCircularCopy)
+                .then(function () {
+                    // The plot renders successfully
+                    expect(gd.calcdata[0][0].circular).toBe(true);
+                    expect(d3SelectAll('.sankey .node-rect').size()).toBeGreaterThan(0);
+
+                    // An error is logged about the fallback
+                    expect(errors.length).toBe(1);
+                })
+                .then(done, done.fail);
+        });
+
         it('can create groups, restyle groups and properly update DOM', function (done) {
             var mockCircularCopy = Lib.extendDeep({}, mockCircular);
             var firstGroup = [

test/plot-schema.json

@@ -50746,6 +50746,16 @@
             "valType": "number"
           },
           "role": "object",
+          "sort": {
+            "description": "If the value is `auto` (the default), the vertical order of nodes will be determined automatically by the layout. If the value is `input`, the vertical order is kept the same as the order in the input node array",
+            "dflt": "auto",
+            "editType": "calc",
+            "valType": "enumerated",
+            "values": [
+              "auto",
+              "input"
+            ]
+          },
           "thickness": {
             "arrayOk": false,
             "description": "Sets the thickness (in px) of the `nodes`.",