From c69752314e2c994e96f0a115792c61b458f6a6fd Mon Sep 17 00:00:00 2001
From: Dan Vanderkam <danvdk@gmail.com>
Date: Sun, 17 Feb 2013 20:13:07 -0500
Subject: [PATCH] Fix and regression test for Issue 147: X-axis date labels
 break after DST transitions

---
 auto_tests/tests/axis_labels.js | 35 ++++++++++++++++++++++++++++++++++-
 dygraph-tickers.js              | 14 +++++++++++++-
 2 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/auto_tests/tests/axis_labels.js b/auto_tests/tests/axis_labels.js
index 787a3c3..a40a1d2 100644
--- a/auto_tests/tests/axis_labels.js
+++ b/auto_tests/tests/axis_labels.js
@@ -728,7 +728,7 @@ AxisLabelsTestCase.prototype.testLabelFormatterOverridesLabelsKMB = function() {
  * This test shows that you can override labelsKMB on the axis level.
  */
 AxisLabelsTestCase.prototype.testLabelsKMBIgnoredWhenOverridden = function() {
-  g = new Dygraph(
+  var g = new Dygraph(
       document.getElementById("graph"),
       "x,a,b\n" +
       "1,0,2000\n" +
@@ -750,3 +750,36 @@ AxisLabelsTestCase.prototype.testLabelsKMBIgnoredWhenOverridden = function() {
   assertEquals(["0","500","1K","1.5K","2K"], Util.getYLabels(1));
   assertEquals(["0","500","1000","1500","2000"], Util.getYLabels(2));
 };
+
+// Regression test for http://code.google.com/p/dygraphs/issues/detail?id=147
+// Checks that axis labels stay sane across a DST change.
+AxisLabelsTestCase.prototype.testLabelsCrossDstChange = function() {
+  // (From tests/daylight-savings.html)
+  var g = new Dygraph(
+      document.getElementById("graph"),
+      "Date/Time,Purchases\n" +
+      "2010-11-05 00:00:00,167082\n" +
+      "2010-11-06 00:00:00,168571\n" +
+      "2010-11-07 00:00:00,177796\n" +
+      "2010-11-08 00:00:00,165587\n" +
+      "2010-11-09 00:00:00,164380\n",
+      { width: 1024 }
+      );
+
+  // Dates and "nice" hours: 6AM/PM and noon, not 5AM/11AM/...
+  var okLabels = {
+    '05Nov': true,
+    '06Nov': true,
+    '07Nov': true,
+    '08Nov': true,
+    '09Nov': true,
+    '06:00': true,
+    '12:00': true,
+    '18:00': true
+  };
+
+  var xLabels = Util.getXLabels();
+  for (var i = 0; i < xLabels.length; i++) {
+    assertTrue(okLabels[xLabels[i]]);
+  }
+};
diff --git a/dygraph-tickers.js b/dygraph-tickers.js
index 0e18394..65355e5 100644
--- a/dygraph-tickers.js
+++ b/dygraph-tickers.js
@@ -375,9 +375,21 @@ Dygraph.getDateAxis = function(start_time, end_time, granularity, opts, dg) {
     }
     start_time = d.getTime();
 
+    var start_offset_min = new Date(start_time).getTimezoneOffset();
     for (t = start_time; t <= end_time; t += spacing) {
+      var d = new Date(t);
+
+      // This ensures that we stay on the same hourly "rhythm" across
+      // daylight savings transitions. Without this, the ticks could get off
+      // by an hour. See tests/daylight-savings.html or issue 147.
+      if (d.getTimezoneOffset() != start_offset_min) {
+        t += (d.getTimezoneOffset() - start_offset_min) * 60 * 1000;
+        d = new Date(t);
+        start_offset_min = d.getTimezoneOffset();
+      }
+
       ticks.push({ v:t,
-                   label: formatter(new Date(t), granularity, opts, dg)
+                   label: formatter(d, granularity, opts, dg)
                  });
     }
   } else {
-- 
2.7.4