New time scale ticks.mode/.source options (#4507)

`ticks.source` (`'auto'`(default)|`'labels'`): `auto` generates "optimal" ticks based on min, max and a few more options (current `time` implementation`). `labels` generates ticks from the user given `data.labels` values (two additional trailing and leading ticks can be added if min and max are provided).

`ticks.mode` (`'linear'`(default)|`series`): `series` displays ticks at the same distance from each other, whatever the time value they represent, while `linear` displays them linearly in time: the distance between each tick represent the amount of time between their time values.

src/helpers/helpers.time.js

@@ -3,6 +3,8 @@
 var moment = require('moment');
 moment = typeof(moment) === 'function' ? moment : window.moment;
 
+var helpers = require('./helpers.core');
+
 var interval = {
 	millisecond: {
 		size: 1,
@@ -53,7 +55,7 @@ function generateTicksNiceRange(options, dataRange, niceRange) {
 	var ticks = [];
 	if (options.maxTicks) {
 		var stepSize = options.stepSize;
-		var startTick = options.min !== undefined ? options.min : niceRange.min;
+		var startTick = helpers.isNullOrUndef(options.min)? niceRange.min : options.min;
 		var majorUnit = options.majorUnit;
 		var majorUnitStart = majorUnit ? moment(startTick).add(1, majorUnit).startOf(majorUnit) : startTick;
 		var startRange = majorUnitStart.valueOf() - startTick;