code: func funcStddevOverTime(vals []Value, args Expressions, enh *EvalNodeHelper) Vector {
  	return aggrOverTime(vals, enh, func(values []Point) float64 {
  		var aux, count, mean float64
  		for _, v := range values {
  			count++
  			delta := v.V - mean
  			mean += delta / count
  			aux += delta * (v.V - mean)
  		}
  		return math.Sqrt(aux / count)
  	})
  }
docs: // === stddev_over_time(Matrix ValueTypeMatrix) Vector ===