Implement code review suggestions

- avoid useless push while building the lookup table
- rename `this._timestamps` to `this._data`

src/scales/scale.time.js

@@ -104,11 +104,9 @@ function buildLookupTable(timestamps, min, max, distribution) {
 	}
 
 	var table = [];
-	var items = [];
+	var items = [min];
 	var i, ilen, prev, curr, next;
 
-	items.push(min);
-
 	for (i = 0, ilen = timestamps.length; i < ilen; ++i) {
 		curr = timestamps[i];
 		if (curr > min && curr < max) {
@@ -507,9 +505,9 @@ module.exports = function(Chart) {
 			me.max = Math.max(min + 1, max);
 
 			// PRIVATE
+			me._data = timestamps;
 			me._datasets = datasets;
 			me._horizontal = me.isHorizontal();
-			me._timestamps = timestamps;
 			me._labels = labels;
 			me._table = [];
 		},
@@ -531,7 +529,7 @@ module.exports = function(Chart) {
 
 			switch (ticksOpts.source) {
 			case 'data':
-				timestamps = me._timestamps;
+				timestamps = me._data;
 				break;
 			case 'labels':
 				timestamps = me._labels;
@@ -566,7 +564,7 @@ module.exports = function(Chart) {
 			me._majorUnit = majorUnit;
 			me._minorFormat = formats[unit];
 			me._majorFormat = formats[majorUnit];
-			me._table = buildLookupTable(me._timestamps, min, max, options.distribution);
+			me._table = buildLookupTable(me._data, min, max, options.distribution);
 
 			return ticksFromTimestamps(ticks, majorUnit);
 		},