Расчет среднего балла по предметам

private void UpdateChart(object sender, SelectionChangedEventArgs e)
    {
        if (TypesOfDiagramCombo.SelectedItem is SeriesChartType currentType)
        {
            Series currentSeries = ChartStudentProgress.Series.FirstOrDefault();
            currentSeries.ChartType = currentType;
            currentSeries.Points.Clear();
            var counter = 0;
            var sum = 0;
            var gradesList = ClassForData.DBEntity.Grades.Where(it => it.ID_Student == users.ID).ToObservableCollection();
            currentSeries.Points.AddXY(currentSubjectGrade.FirstOrDefault().Subjects.Title, AverageGrades.ToString());
           
            foreach (var item in gradesList)
            {
                counter++;
                sum += item.Grade.Value;
            }
            AverageGrades = sum / counter;
 
            foreach (var item in gradesList)
            {
                currentSeries.Points.AddXY(item.Subjects.Title, AverageGrades.ToString());
            }
        }
    }

currentSeries.Points.AddXY(item.Subjects.Title, AverageGrades.ToString());

 string connectionString = "Data Source=DESKTOP-J9KRE1V;Initial Catalog=Electronic_diary;Integrated Security=True";
                string sql = $"SELECT SUM(Grade)/CONVERT(float, COUNT(Grade), 2), Title FROM Grades g left join Subjects s on g.[ID Subject] = s.[ID Subject] WHERE [ID Student] = {users.ID} GROUP BY s.Title";
                try
                {
                    using (SqlConnection sqlConnection = new SqlConnection(connectionString))
                    {
                        sqlConnection.Open();
                        SqlCommand command = new SqlCommand(sql, sqlConnection);
                        SqlDataReader reader = command.ExecuteReader();
                        if (reader.HasRows)
                        {
                            while (reader.Read())
                            {
                                AverageGrades = (int)reader.GetFloat(0);
                                SubjectTitles = reader.GetString(1);
                                currentSeries.Points.AddXY(SubjectTitles, AverageGrades.ToString());
                            }
                        }
                        reader.Close();
                    }
                }
                catch (SqlException ex)
                {
                    new CustomMessageBox(ex.Message.ToString(), CustomMessageBox.MessageType.Error, CustomMessageBox.MessageButtons.Ok).ShowDialog();
                }

using (SqlDataReader sqlDataReader = await sqlCommand.ExecuteReaderAsync())
{
    while (await sqlDataReader.ReadAsync())
    {
        drawSpecifications.Add(new DrawSpecification
        {
            DrawSpecID = (int)sqlDataReader["ID_DrawSpec"],
            DrawSystemID = (int)sqlDataReader["ID_DrawSys"],
            DSPositionNum = (short)sqlDataReader["SpecPos"],
            DSKKS = sqlDataReader["KKS"] as string ?? "",
            DSDesignation = sqlDataReader["Designation"] as string ?? "",
            DSName = sqlDataReader["SName"] as string ?? "",
            DSQuantity = sqlDataReader["Quantity"] ==
                DBNull.Value ? null : (double?)sqlDataReader["Quantity"],
            DSQuantityAssembly = sqlDataReader["QuantityA"] ==
                DBNull.Value ? null : (double?)sqlDataReader["QuantityA"],
            DSMeasure = sqlDataReader["Measure"] as string ?? "",
            DSMaterial = sqlDataReader["Material"] as string ?? "",
            DSUnitWeight = sqlDataReader["UnitWeight"] ==
                DBNull.Value ? null : (double?)sqlDataReader["UnitWeight"],
            DSWorkOrder = sqlDataReader["ID_WorkOrder"] ==
                DBNull.Value ? null : (int?)sqlDataReader["ID_WorkOrder"],
            DSWorkOrderAssembly = sqlDataReader["ID_WorkOrderA"] ==
                DBNull.Value ? null : (int?)sqlDataReader["ID_WorkOrderA"]
        });
    }
}