code: function getAllowedSteps(gradation, units)
  {
  	return gradation.filter(({ unit }) => {
  		// If this step has a `unit` defined
  		// then this `unit` must be in the list of `units` allowed.
  		if (unit) {
  			return units.indexOf(unit) >= 0
  		}
  		// A gradation step is not required to specify a `unit`.
  		// E.g. for Twitter gradation it specifies `format()` instead.
  		return true
  	})
  }
docs: Leaves only allowed gradation steps.
  @param  {Object[]} gradation
  @param  {string[]} units - Allowed time units.
  @return {Object[]}