From c09ce2eae1bee66b26d6ed4ccb362dc9ea483370 Mon Sep 17 00:00:00 2001
From: Jyrki Niemi <jyrki.niemi@helsinki.fi>
Date: Tue, 25 May 2021 16:14:41 +0300
Subject: [PATCH] Fix #38: Correct trend diagram labels for decades <=1900

results.js: view.GraphResults.renderGraph:
- Render correct x-axis decade labels in the trend diagram for decades
  1900 and before by passing to Rickshaw.Graph.Axis.Time constructor the
  existing fixed time.ceil function with special handling of decades.
  (Previously, labels for decades 1900 and before were shifted back by
  one (1890 instead of 1900 and so on), probably resulting from
  Rickshaw's approximate handling of leap years.)
- Modify the fixed time.ceil to work for decades 1800 and before.
---
 app/scripts/results.js | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/app/scripts/results.js b/app/scripts/results.js
index b015df35e..e59b61998 100644
--- a/app/scripts/results.js
+++ b/app/scripts/results.js
@@ -2156,13 +2156,21 @@ view.GraphResults = class GraphResults extends BaseResults {
         const toDate = (sec) => moment(sec * 1000).toDate()
 
         const time = new Rickshaw.Fixtures.Time()
+        // Fix time.ceil for decades: Rickshaw.Fixtures.Time.ceil
+        // returns one decade too small values for 1900 and before.
+        // (The root cause may be Rickshaw's approximate handling of
+        // leap years: 1900 was not a leap year.)
         const old_ceil = time.ceil
         time.ceil = (time, unit) => {
             if (unit.name === "decade") {
                 const out = Math.ceil(time / unit.seconds) * unit.seconds
                 const mom = moment(out * 1000)
-                if (mom.date() === 31) {
-                    mom.add("day", 1)
+                const monthDay = mom.date()
+                // If the day of the month is not 1, it is within the
+                // previous month (December), so add enough days to
+                // move the date to the expected month (January).
+                if (monthDay !== 1) {
+                    mom.add(32 - monthDay, "day")
                 }
                 return mom.unix()
             } else {
@@ -2172,9 +2180,10 @@ view.GraphResults = class GraphResults extends BaseResults {
 
         const xAxis = new Rickshaw.Graph.Axis.Time({
             graph,
+            // Use the fixed .ceil for decades
+            timeFixture: time,
+            // timeUnit: time.unit("month") # TODO: bring back decade
         })
-        // timeUnit: time.unit("month") # TODO: bring back decade
-        // timeFixture: new Rickshaw.Fixtures.Time()
 
         this.preview = new Rickshaw.Graph.RangeSlider.Preview({
             graph,