Fix geo projections losing their D3 default rotation

Geo.updateProjection unconditionally called
projection.rotate([-rotation.lon, -rotation.lat, rotation.roll]), so the
default projection.rotation attributes ([0, 0, 0]) discarded the rotation
the d3 projection factory ships with. Six projections are affected
(albers, bertin1953, gringorten and peirce quincuncial, sinu mollweide,
wiechel).

Capture the factory rotation when the projection wrapper is created and
compose the user rotation on top of it. Projections whose d3 default is
the identity are bit-for-bit unchanged; an explicit rotation now rotates
relative to the projection's canonical orientation instead of replacing
it.

Fixes #7949

Author: CAOShurong

src/plots/geo/geo.js

@@ -304,7 +304,11 @@ proto.updateProjection = function(geoCalcData, fullLayout) {
     // set 'pre-fit' projection
     projection
         .center([center.lon - rotation.lon, center.lat - rotation.lat])
-        .rotate([-rotation.lon, -rotation.lat, rotation.roll])
+        .rotate([
+            -rotation.lon + projection.defaultRotation[0],
+            -rotation.lat + projection.defaultRotation[1],
+            rotation.roll + projection.defaultRotation[2]
+        ])
         .parallels(projLayout.parallels);
 
     // fit projection 'scale' and 'translate' to set lon/lat ranges
@@ -716,6 +720,12 @@ function getProjection(geoLayout) {
     projName = 'geo' + Lib.titleCase(projName);
     var projFn = geo[projName] || geoProjection[projName];
     var projection = projFn();
+    // Capture the rotation the projection factory ships with (identity for
+    // most projections, but e.g. albers defaults to [96, 0, 0]) so that the
+    // default projection.rotation attributes do not discard the canonical
+    // orientation. See #7949.
+    var projDefaultRotation = typeof projection.rotate === 'function' ?
+        projection.rotate() : [0, 0, 0];
 
     var clipAngle =
         geoLayout._isSatellite ? Math.acos(1 / projLayout.distance) * 180 / Math.PI :
@@ -754,6 +764,8 @@ function getProjection(geoLayout) {
         return projection.getPath().bounds(object);
     };
 
+    projection.defaultRotation = projDefaultRotation;
+
     projection.precision(constants.precision);
 
     if(geoLayout._isSatellite) {

test/jasmine/tests/geo_test.js

@@ -2876,3 +2876,37 @@ describe('plotly_relayouting', function() {
         });
     });
 });
+
+describe('Test geo projection D3 default rotation (#7949)', function() {
+    var gd;
+
+    beforeEach(function() { gd = createGraphDiv(); });
+    afterEach(destroyGraphDiv);
+
+    function subplot() { return gd._fullLayout.geo._subplot; }
+
+    it('preserves the projection factory rotation at default projection.rotation', function(done) {
+        Plotly.newPlot(gd, [{ type: 'scattergeo', lon: [], lat: [] }], {
+            geo: { projection: { type: 'albers' } }
+        }).then(function() {
+            // d3-geo albers ships with rotation [96, 0, 0]; plotly's default
+            // projection.rotation attributes must not discard it
+            expect(subplot().projection.defaultRotation).toEqual([96, 0, 0]);
+            expect(subplot().projection.rotate()).toEqual([96, 0, 0]);
+        }).then(function() {
+            return Plotly.relayout(gd, { 'geo.projection.rotation.lon': 10 });
+        }).then(function() {
+            // user rotation composes on top of the factory rotation
+            expect(subplot().projection.rotate()[0]).toBe(96 - 10);
+        }).then(done, done.fail);
+    });
+
+    it('keeps identity-rotation projections exactly as before', function(done) {
+        Plotly.newPlot(gd, [{ type: 'scattergeo', lon: [], lat: [] }], {
+            geo: { projection: { type: 'mercator' } }
+        }).then(function() {
+            expect(subplot().projection.defaultRotation).toEqual([0, 0, 0]);
+            expect(subplot().projection.rotate()).toEqual([0, 0, 0]);
+        }).then(done, done.fail);
+    });
+});