src/scales/scale.time.js
function determineUnitForFormatting(scale, ticks, minUnit, min, max) {
	var ilen = UNITS.length;
	var i, unit;

	for (i = ilen - 1; i >= UNITS.indexOf(minUnit); i--) {
		unit = UNITS[i];
		if (INTERVALS[unit].common && scale._adapter.diff(max, min, unit) >= ticks.length) {
			return unit;
		}
	}

	return UNITS[minUnit ? UNITS.indexOf(minUnit) : 0];
}
function determineUnitForFormatting(scale, ticks, minUnit, min, max) {
	var ilen = UNITS.length;
	var i, unit;

	for (i = ilen - 1; i >= UNITS.indexOf(minUnit); i--) {
		unit = UNITS[i];
		if (INTERVALS[unit].common && scale._adapter.diff(max, min, unit) >= ticks.length) {
			return unit;
		}
	}

	return UNITS[minUnit ? UNITS.indexOf(minUnit) : 0];
}
function determineUnitForFormatting ( scale , ticks , minUnit , min , max ) { var ilen = UNITS . length ; var i , unit ; for ( i = ilen - 1 ; i >= UNITS . indexOf ( minUnit ) ; i -- ) { unit = UNITS [ i ] ; if ( INTERVALS [ unit ] . common && scale . _adapter . diff ( max , min , unit ) >= ticks . length ) { return unit ; } } return UNITS [ minUnit ? UNITS . indexOf ( minUnit ) : 0 ] ; }
Figures out what unit to format a set of ticks with
Figures out what unit to format a set of ticks with