use a Sunday instead of Monday for weekly period0

Author: archmoj

src/traces/scatter/attributes.js

@@ -43,7 +43,7 @@ function axisPeriod0(axis) {
             'Only relevant when the axis `type` is *date*.',
             'Sets the base for period positioning in milliseconds or date string on the ' + axis + ' axis.',
             'When `' + axis + 'period` is round number of weeks,',
-            'the `' + axis + 'period0` by default would be on a Monday i.e. 1970-01-05,',
+            'the `' + axis + 'period0` by default would be on a Sunday i.e. 1970-01-04,',
             'otherwise it would be at 1970-01-01.'
         ].join(' ')
     };

src/traces/scatter/period_defaults.js

@@ -15,7 +15,7 @@ function getPeriod0Dflt(period) {
     var n = period / ONEWEEK;
 
     return Math.round(n) === n ?
-        '1970-01-05' : // a Monday
+        '1970-01-04' : // a Sunday
         '1970-01-01';
 }
 

test/image/baselines/period_positioning8.png

@@ -14,20 +14,20 @@
             "hovertemplate": "%{x|%A}, %{y}",
             "type": "bar",
             "y": [
-                21,
-                21,
-                21,
-                21,
+                28,
+                28,
+                28,
                 12,
                 12,
                 12,
                 12,
                 12,
                 12,
                 12,
-                28,
-                28,
-                28
+                21,
+                21,
+                21,
+                21
             ],
             "x": [
                 "2020-01-16",