Adjust tick0 for weekly time periods

src/lib/dates.js

@@ -45,17 +45,31 @@ function isWorldCalendar(calendar) {
 /*
  * dateTick0: get the canonical tick for this calendar
  *
+ * integer weekdays : Saturday: 0, Sunday: 1, Monday: 2, etc.
+ */
+exports.dateTick0 = function(calendar, dayOfWeek) {
+    var tick0 = _dateTick0(calendar, !!dayOfWeek);
+    if(dayOfWeek < 2) return tick0;
+
+    var v = exports.dateTime2ms(tick0, calendar);
+    v += ONEDAY * (dayOfWeek - 1); // shift Sunday to Monday, etc.
+    return exports.ms2DateTime(v, 0, calendar);
+};
+
+/*
+ * _dateTick0: get the canonical tick for this calendar
+ *
  * bool sunday is for week ticks, shift it to a Sunday.
  */
-exports.dateTick0 = function(calendar, sunday) {
+function _dateTick0(calendar, sunday) {
     if(isWorldCalendar(calendar)) {
         return sunday ?
             Registry.getComponentMethod('calendars', 'CANONICAL_SUNDAY')[calendar] :
             Registry.getComponentMethod('calendars', 'CANONICAL_TICK')[calendar];
     } else {
         return sunday ? '2000-01-02' : '2000-01-01';
     }
-};
+}
 
 /*
  * dfltRange: for each calendar, give a valid default range

src/plots/cartesian/axes.js

@@ -630,12 +630,14 @@ axes.calcTicks = function calcTicks(ax, opts) {
     generateTicks();
 
     var isPeriod = ax.ticklabelmode === 'period';
-    if(isPeriod) {
+    var addedPreTick0Label = false;
+    if(isPeriod && tickVals[0]) {
         // add one label to show pre tick0 period
         tickVals.unshift({
             minor: false,
             value: axes.tickIncrement(tickVals[0].value, ax.dtick, !axrev, ax.caldendar)
         });
+        addedPreTick0Label = true;
     }
 
     if(ax.rangebreaks) {
@@ -778,7 +780,7 @@ axes.calcTicks = function calcTicks(ax, opts) {
         ticksOut.push(t);
     }
 
-    if(isPeriod) {
+    if(isPeriod && addedPreTick0Label) {
         var removedPreTick0Label = false;
 
         for(i = 0; i < ticksOut.length; i++) {
@@ -965,7 +967,7 @@ axes.autoTicks = function(ax, roughDTick) {
     }
 
     if(ax.type === 'date') {
-        ax.tick0 = Lib.dateTick0(ax.calendar);
+        ax.tick0 = Lib.dateTick0(ax.calendar, 0);
         // the criteria below are all based on the rough spacing we calculate
         // being > half of the final unit - so precalculate twice the rough val
         var roughX2 = 2 * roughDTick;
@@ -982,14 +984,11 @@ axes.autoTicks = function(ax, roughDTick) {
             // get week ticks on sunday
             // this will also move the base tick off 2000-01-01 if dtick is
             // 2 or 3 days... but that's a weird enough case that we'll ignore it.
-            ax.tick0 = Lib.dateTick0(ax.calendar, true);
-
             var tickformat = axes.getTickFormat(ax);
             if(/%[uVW]/.test(tickformat)) {
-                // replace Sunday with Monday for ISO and Monday-based formats
-                var len = ax.tick0.length;
-                var lastD = +ax.tick0[len - 1];
-                ax.tick0 = ax.tick0.substring(0, len - 2) + String(lastD + 1);
+                ax.tick0 = Lib.dateTick0(ax.calendar, 2); // Monday
+            } else {
+                ax.tick0 = Lib.dateTick0(ax.calendar, 1); // Sunday
             }
         } else if(roughX2 > ONEHOUR) {
             ax.dtick = roundDTick(roughDTick, ONEHOUR, roundBase24);

src/plots/cartesian/clean_ticks.js

@@ -10,7 +10,9 @@
 
 var isNumeric = require('fast-isnumeric');
 var Lib = require('../../lib');
-var ONEDAY = require('../../constants/numerical').ONEDAY;
+var constants = require('../../constants/numerical');
+var ONEDAY = constants.ONEDAY;
+var ONEWEEK = constants.ONEWEEK;
 
 /**
  * Return a validated dtick value for this axis
@@ -75,7 +77,9 @@ exports.dtick = function(dtick, axType) {
  */
 exports.tick0 = function(tick0, axType, calendar, dtick) {
     if(axType === 'date') {
-        return Lib.cleanDate(tick0, Lib.dateTick0(calendar));
+        return Lib.cleanDate(tick0,
+            Lib.dateTick0(calendar, (dtick % ONEWEEK === 0) ? 1 : 0)
+        );
     }
     if(dtick === 'D1' || dtick === 'D2') {
         // D1 and D2 modes ignore tick0 entirely

src/traces/scatter/period_defaults.js

@@ -13,8 +13,10 @@ var numConstants = require('../../constants/numerical');
 var ONEWEEK = numConstants.ONEWEEK;
 
 function getPeriod0Dflt(period, calendar) {
-    var n = period / ONEWEEK;
-    return dateTick0(calendar, Math.round(n) === n);
+    if(period % ONEWEEK === 0) {
+        return dateTick0(calendar, 1); // Sunday
+    }
+    return dateTick0(calendar, 0);
 }
 
 module.exports = function handlePeriodDefaults(traceIn, traceOut, layout, coerce, opts) {

test/jasmine/tests/axes_test.js

@@ -13,8 +13,10 @@ var Cartesian = require('@src/plots/cartesian');
 var Axes = require('@src/plots/cartesian/axes');
 var Fx = require('@src/components/fx');
 var supplyLayoutDefaults = require('@src/plots/cartesian/layout_defaults');
-var BADNUM = require('@src/constants/numerical').BADNUM;
-var ONEDAY = require('@src/constants/numerical').ONEDAY;
+var numerical = require('@src/constants/numerical');
+var BADNUM = numerical.BADNUM;
+var ONEDAY = numerical.ONEDAY;
+var ONEWEEK = numerical.ONEWEEK;
 
 var createGraphDiv = require('../assets/create_graph_div');
 var destroyGraphDiv = require('../assets/destroy_graph_div');
@@ -6020,6 +6022,42 @@ describe('Test axes', function() {
                 .then(done);
             });
         });
+
+        [undefined, '%U', '%V', '%W'].forEach(function(tickformat) {
+            it('with ' + tickformat + ' tickformat, should default tick0 on a Sunday when dtick is a round number of weeks', function(done) {
+                var fig = {
+                    data: [
+                        {
+                            showlegend: false,
+                            type: 'bar',
+                            width: ONEWEEK,
+                            xperiod: ONEWEEK,
+                            x: [
+                                '2020-09-16',
+                                '2020-09-24',
+                                '2020-09-30'
+                            ],
+                            y: [3, 2, 4]
+                        }
+                    ],
+                    layout: {
+                        xaxis: {
+                            tickformat: tickformat,
+                            dtick: ONEWEEK,
+                            ticklabelmode: 'period',
+                            showgrid: true,
+                        }
+                    }
+                };
+
+                Plotly.newPlot(gd, fig)
+                .then(function() {
+                    expect(gd._fullLayout.xaxis.tick0).toBe('2000-01-02');
+                })
+                .catch(failTest)
+                .then(done);
+            });
+        });
     });
 });