Excel TREND Function in C#

ExcelTrend.cs

public static class StatisticalCalculation
    {
        public static double Trend(IEnumerable<double> knownYValues, IEnumerable<double> knownXValues, double newX)
        {
            var line = LineByLeastSquares(knownYValues, knownXValues);
            return line.slope * newX + line.intercept;
        }

        private static (double slope, double intercept) LineByLeastSquares(IEnumerable<double> knownYValues, IEnumerable<double> knownXValues)
        {
            (double xAvg, List<double> xValues) = NormalizeAndFindAvg(knownXValues);
            (double yAvg, List<double> yValues) = NormalizeAndFindAvg(knownYValues);

            if (xValues.Count != yValues.Count)
                throw new ArgumentException($"{nameof(yValues)} must match {nameof(xValues)} [{yValues.Count}, {xValues.Count}].");
            
            double dividendSum = 0d;
            double divisorSum = 0d;

            for (int i = 0; i < yValues.Count; ++i)
            {
                double xValue = xValues[i];
                double yValue = yValues[i];
                double xDiff = xValue - xAvg;
                dividendSum += xDiff * (yValue - yAvg);
                divisorSum += Math.Pow(xDiff, 2);
            }

            double slope = dividendSum / divisorSum;

            return (slope, yAvg - (slope * xAvg));
        }

        private static (double avg, List<double> valueList) NormalizeAndFindAvg(IEnumerable<double> values)
        {
            double sum = 0d;
            var listValues = new List<double>();
            foreach (var value in values)
            {
                listValues.Add(value);
                sum += value;
            }

            return (sum / listValues.Count, listValues);
        }
    }