From ebad5b91db53dba5eff9d3f8ebdff54d037b77d5 Mon Sep 17 00:00:00 2001
From: contactjawad <contact.jawaddd@gmail.com>
Date: Tue, 25 Aug 2026 11:47:19 +0500
Subject: [PATCH] fix(scale): detect even major-tick spacing when first major
 is at index 0

getEvenSpacing seeded the reference gap with arr[0] (an absolute index)
and started comparing at i=1, so any evenly-spaced set of major ticks
whose first index differs from the spacing (e.g. majors at 0, 20, 40)
was wrongly reported as unevenly spaced. Seed the gap with the first
actual difference (arr[1] - arr[0]) and start at i=2.
---
 src/core/core.scale.autoskip.js |  2 +-
 test/specs/scale.time.tests.js  | 41 +++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/src/core/core.scale.autoskip.js b/src/core/core.scale.autoskip.js
index b703bda85a9..ea6d1149067 100644
--- a/src/core/core.scale.autoskip.js
+++ b/src/core/core.scale.autoskip.js
@@ -161,7 +161,7 @@ function getEvenSpacing(arr) {
     return false;
   }
 
-  for (diff = arr[0], i = 1; i < len; ++i) {
+  for (diff = arr[1] - arr[0], i = 2; i < len; ++i) {
     if (arr[i] - arr[i - 1] !== diff) {
       return false;
     }
diff --git a/test/specs/scale.time.tests.js b/test/specs/scale.time.tests.js
index 42817ae15c9..125b289cc14 100644
--- a/test/specs/scale.time.tests.js
+++ b/test/specs/scale.time.tests.js
@@ -1160,6 +1160,47 @@ describe('Time scale tests', function() {
     expect(scale.getPixelForDecimal(1.0)).toBeCloseToPixel(512);
   });
 
+  it('should divide evenly spaced major ticks starting at index 0 into even chunks when autoSkipping', function() {
+    // Hourly data starting on a day boundary: major ticks (day starts) fall at
+    // indices 0, 24, 48, ... i.e. evenly spaced but the first major is at index 0.
+    var data = [];
+    var date = moment('2020-01-01T00:00:00');
+    for (var i = 0; i < 144; i++) {
+      data.push({x: date.valueOf(), y: i});
+      date = date.clone().add(1, 'hour');
+    }
+
+    var chart = window.acquireChart({
+      type: 'line',
+      data: {datasets: [{data: data}]},
+      options: {
+        scales: {
+          x: {
+            type: 'time',
+            time: {unit: 'hour'},
+            ticks: {
+              source: 'data',
+              autoSkip: true,
+              maxTicksLimit: 24,
+              major: {enabled: true},
+              maxRotation: 0
+            }
+          }
+        }
+      }
+    }, {canvas: {width: 4000, height: 150}});
+
+    var values = chart.scales.x.ticks.map(t => t.value);
+
+    // The kept minor ticks should be spaced by a factor of the 24h major interval
+    // (8 hours here). Before the fix the even-major spacing was not detected, so the
+    // spacing fell back to the raw ticks/limit ratio (6 hours), keeping 24 ticks.
+    expect(values.length).toEqual(18);
+    for (var j = 1; j < values.length; j++) {
+      expect(values[j] - values[j - 1]).toEqual(8 * 3600000);
+    }
+  });
+
   ['data', 'labels'].forEach(function(source) {
     ['timeseries', 'time'].forEach(function(type) {
       describe('when ticks.source is "' + source + '" and scale type is "' + type + '"', function() {