Fix choosing of formatting unit

src/scales/scale.time.js

@@ -253,7 +253,10 @@ function determineStepSize(min, max, unit, capacity) {
 	return factor;
 }
 
-function determineUnit(minUnit, min, max, capacity) {
+/**
+ * Figures out what unit results in an appropriate number of auto-generated ticks
+ */
+function determineUnitForAutoTicks(minUnit, min, max, capacity) {
 	var ilen = UNITS.length;
 	var i, interval, factor;
 
@@ -269,6 +272,24 @@ function determineUnit(minUnit, min, max, capacity) {
 	return UNITS[ilen - 1];
 }
 
+/**
+ * Figures out what unit to format a set of ticks with
+ */
+function determineUnitForFormatting(ticks, minUnit, min, max) {
+	var duration = moment.duration(moment(max).diff(moment(min)));
+	var ilen = UNITS.length;
+	var i, unit;
+
+	for (i = ilen - 1; i >= UNITS.indexOf(minUnit); i--) {
+		unit = UNITS[i];
+		if (INTERVALS[unit].major && duration.as(unit) >= ticks.length) {
+			return unit;
+		}
+	}
+
+	return UNITS[minUnit ? UNITS.indexOf(minUnit) : 0];
+}
+
 function determineMajorUnit(unit) {
 	for (var i = UNITS.indexOf(unit) + 1, ilen = UNITS.length; i < ilen; ++i) {
 		if (INTERVALS[UNITS[i]].major) {
@@ -283,8 +304,10 @@ function determineMajorUnit(unit) {
  * Important: this method can return ticks outside the min and max range, it's the
  * responsibility of the calling code to clamp values if needed.
  */
-function generate(min, max, minor, major, capacity, options) {
+function generate(min, max, capacity, options) {
 	var timeOpts = options.time;
+	var minor = timeOpts.unit || determineUnitForAutoTicks(timeOpts.minUnit, min, max, capacity);
+	var major = determineMajorUnit(minor);
 	var stepSize = helpers.valueOrDefault(timeOpts.stepSize, timeOpts.unitStepSize);
 	var weekday = minor === 'week' ? timeOpts.isoWeekday : false;
 	var majorTicksEnabled = options.ticks.major.enabled;
@@ -553,9 +576,6 @@ module.exports = function(Chart) {
 			var max = me.max;
 			var options = me.options;
 			var timeOpts = options.time;
-			var capacity = me.getLabelCapacity(min);
-			var unit = timeOpts.unit || determineUnit(timeOpts.minUnit, min, max, capacity);
-			var majorUnit = determineMajorUnit(unit);
 			var timestamps = [];
 			var ticks = [];
 			var i, ilen, timestamp;
@@ -569,7 +589,7 @@ module.exports = function(Chart) {
 				break;
 			case 'auto':
 			default:
-				timestamps = generate(min, max, unit, majorUnit, capacity, options);
+				timestamps = generate(min, max, me.getLabelCapacity(min), options);
 			}
 
 			if (options.bounds === 'ticks' && timestamps.length) {
@@ -593,12 +613,12 @@ module.exports = function(Chart) {
 			me.max = max;
 
 			// PRIVATE
-			me._unit = unit;
-			me._majorUnit = majorUnit;
+			me._unit = determineUnitForFormatting(ticks, timeOpts.minUnit, me.min, me.max);
+			me._majorUnit = determineMajorUnit(me._unit);
 			me._table = buildLookupTable(me._timestamps.data, min, max, options.distribution);
 			me._offsets = computeOffsets(me._table, ticks, min, max, options);
 
-			return ticksFromTimestamps(ticks, majorUnit);
+			return ticksFromTimestamps(ticks, me._majorUnit);
 		},
 
 		getLabelForIndex: function(index, datasetIndex) {
@@ -630,7 +650,7 @@ module.exports = function(Chart) {
 			var minorFormat = formats[me._unit];
 			var majorUnit = me._majorUnit;
 			var majorFormat = formats[majorUnit];
-			var majorTime = tick.clone().startOf(me._majorUnit).valueOf();
+			var majorTime = tick.clone().startOf(majorUnit).valueOf();
 			var majorTickOpts = options.ticks.major;
 			var major = majorTickOpts.enabled && majorUnit && majorFormat && time === majorTime;
 			var label = tick.format(formatOverride ? formatOverride : major ? majorFormat : minorFormat);