Fix geo fitbounds to pick compact range across the antimeridian

When `fitbounds` point data straddles +/-180 degrees longitude, the naive
[min, max] range from getAutoRange includes the large empty span the long way
round the globe, so the map zooms out far more than necessary
(plotly/plotly.py#5539).

Add getFitboundsLonRange, which finds the widest gap between consecutive
longitudes and returns the complementary, antimeridian-crossing range when it
is more compact. The override is scoped to longitude point data: it is skipped
when a choropleth or location-based scattergeo trace is present (whose region
extents are not captured here) and when the data spans the whole globe.

Author: SharadhNaidu

src/plots/geo/geo.js

@@ -24,6 +24,7 @@ var selectOnClick = require('../../components/selections').selectOnClick;
 
 var createGeoZoom = require('./zoom');
 var constants = require('./constants');
+var getFitboundsLonRange = require('./get_fitbounds_lon_range');
 
 var geoUtils = require('../../lib/geo_location_utils');
 var topojsonUtils = require('../../lib/topojson_utils');
@@ -233,6 +234,37 @@ proto.updateProjection = function(geoCalcData, fullLayout) {
         axLon.range = getAutoRange(gd, axLon);
         axLat.range = getAutoRange(gd, axLat);
 
+        // For point data straddling the antimeridian (±180°), the naive [min, max]
+        // longitude range above can include a large empty span; prefer the compact
+        // crossing range instead. Skipped when a trace contributes region extents
+        // (choropleth or location-based scattergeo), whose geographic bounds are not
+        // captured by the point longitudes gathered here.
+        if(!this.hasChoropleth && geoLayout.fitbounds === 'locations') {
+            var lons = [];
+            var hasLocationData = false;
+
+            for(var i = 0; i < geoCalcData.length; i++) {
+                var calcTrace = geoCalcData[i];
+                var fitTrace = calcTrace[0].trace;
+
+                // only visible traces contribute to the autorange above
+                if(fitTrace.visible !== true) continue;
+                if(fitTrace.locations) {
+                    hasLocationData = true;
+                    break;
+                }
+                for(var j = 0; j < calcTrace.length; j++) {
+                    var lonlat = calcTrace[j].lonlat;
+                    if(lonlat) lons.push(lonlat[0]);
+                }
+            }
+
+            if(!hasLocationData) {
+                var fitLonRange = getFitboundsLonRange(lons);
+                if(fitLonRange) axLon.range = fitLonRange;
+            }
+        }
+
         var midLon = (axLon.range[0] + axLon.range[1]) / 2;
         var midLat = (axLat.range[0] + axLat.range[1]) / 2;
 

src/plots/geo/get_fitbounds_lon_range.js

@@ -0,0 +1,53 @@
+'use strict';
+
+/**
+ * Pick a compact longitude range for `fitbounds` when the data straddles the
+ * antimeridian (±180°).
+ *
+ * Longitude is cyclic, so the naive [min, max] range used by the autorange
+ * machinery can include a large empty span when points sit on both sides of
+ * ±180° (e.g. lon = [131.8855, -179] spans ~311° the long way round, when the
+ * compact view spans ~49° across the antimeridian). This finds the largest gap
+ * between consecutive longitudes and, when that gap is wider than the gap across
+ * the antimeridian, returns the complementary range so the map shows the dense
+ * cluster of points rather than the empty ocean between them.
+ *
+ * The returned upper bound may exceed 180°; downstream `makeRangeBox` already
+ * handles longitudes that cross the antimeridian without ambiguity.
+ *
+ * @param {Array} lons : longitude values (may contain non-finite entries)
+ * @return {Array|null} [lonStart, lonEnd] when an antimeridian-crossing range is
+ *   more compact, otherwise null (caller keeps the autorange result).
+ */
+module.exports = function getFitboundsLonRange(lons) {
+    var sorted = [];
+    for(var k = 0; k < lons.length; k++) {
+        if(isFinite(lons[k])) sorted.push(lons[k]);
+    }
+    if(sorted.length < 2) return null;
+
+    sorted.sort(function(a, b) { return a - b; });
+
+    var n = sorted.length;
+    var naiveSpan = sorted[n - 1] - sorted[0];
+    // Data already wraps the whole globe; there is nothing to compact.
+    if(naiveSpan >= 360) return null;
+
+    // Widest gap between consecutive longitudes.
+    var maxGap = -Infinity;
+    var gapStart = -1;
+    for(var i = 0; i < n - 1; i++) {
+        var gap = sorted[i + 1] - sorted[i];
+        if(gap > maxGap) {
+            maxGap = gap;
+            gapStart = i;
+        }
+    }
+
+    // Only worth wrapping when an interior gap is wider than the gap that the
+    // naive [min, max] range already leaves open across the antimeridian.
+    var antimeridianGap = 360 - naiveSpan;
+    if(maxGap <= antimeridianGap) return null;
+
+    return [sorted[gapStart + 1], sorted[gapStart] + 360];
+};

test/jasmine/tests/geo_test.js

@@ -4,6 +4,7 @@ var Lib = require('../../../src/lib');
 var Geo = require('../../../src/plots/geo');
 var GeoAssets = require('../../../src/assets/geo_assets');
 var constants = require('../../../src/plots/geo/constants');
+var getFitboundsLonRange = require('../../../src/plots/geo/get_fitbounds_lon_range');
 var geoLocationUtils = require('../../../src/lib/geo_location_utils');
 var topojsonUtils = require('../../../src/lib/topojson_utils');
 
@@ -36,6 +37,75 @@ function move(fromX, fromY, toX, toY, delay) {
     });
 }
 
+describe('Test geo fitbounds longitude range', function() {
+    it('returns the compact crossing range when point data straddles the antimeridian', function() {
+        expect(getFitboundsLonRange([131.8855, -179])).toEqual([131.8855, 181]);
+        expect(getFitboundsLonRange([170, 175, -170])).toEqual([170, 190]);
+    });
+
+    it('keeps the naive range (null) when the data does not straddle the antimeridian', function() {
+        expect(getFitboundsLonRange([131.8855, 179])).toBe(null);
+        expect(getFitboundsLonRange([-10, 0, 20])).toBe(null);
+    });
+
+    it('keeps the naive range (null) when the data spans the whole globe', function() {
+        var lons = [];
+        for(var lon = 0; lon <= 360; lon += 2.5) lons.push(lon);
+        expect(getFitboundsLonRange(lons)).toBe(null);
+    });
+
+    it('returns null when fewer than two finite longitudes are available', function() {
+        expect(getFitboundsLonRange([10])).toBe(null);
+        expect(getFitboundsLonRange([NaN, 5])).toBe(null);
+        expect(getFitboundsLonRange([])).toBe(null);
+    });
+});
+
+describe('Test geo fitbounds with antimeridian-straddling points', function() {
+    var gd;
+
+    beforeEach(function() { gd = createGraphDiv(); });
+
+    afterEach(destroyGraphDiv);
+
+    function _plot(lons) {
+        return Plotly.newPlot(gd, [{
+            type: 'scattergeo',
+            mode: 'markers',
+            lat: [43.1155, 32.7157],
+            lon: lons
+        }], {
+            geo: {fitbounds: 'locations', projection: {type: 'equirectangular'}},
+            width: 700,
+            height: 500
+        });
+    }
+
+    it('centers on the compact crossing view when points straddle the antimeridian', function(done) {
+        // lon = [131.8855, -179] spans ~311deg the naive way; the compact view
+        // crosses the antimeridian, giving range [131.8855, 181] and a projection
+        // rotated to its mid-longitude (~156.4deg), not to the naive mid (~-24deg).
+        _plot([131.8855, -179]).then(function() {
+            var geoLayout = gd._fullLayout.geo;
+            expect(geoLayout.lonaxis._ax.range).toEqual([131.8855, 181]);
+            expect(geoLayout._subplot.projection.rotate()[0]).toBeCloseTo(-156.4, 0);
+        })
+        .then(done, done.fail);
+    });
+
+    it('keeps the naive centering when points do not straddle the antimeridian', function(done) {
+        _plot([131.8855, 179]).then(function() {
+            var geoLayout = gd._fullLayout.geo;
+            // projection rotated to the naive mid-longitude (~155.4deg); the range is
+            // not wrapped across the antimeridian (which would rotate near -24deg or +156deg)
+            var rotateLon = geoLayout._subplot.projection.rotate()[0];
+            expect(rotateLon).toBeLessThan(-150);
+            expect(rotateLon).toBeGreaterThan(-160);
+        })
+        .then(done, done.fail);
+    });
+});
+
 describe('Test Geo layout defaults', function() {
     var layoutAttributes = Geo.layoutAttributes;
     var supplyLayoutDefaults = Geo.supplyLayoutDefaults;