積層棒グラフの棒の上部に合計値を表示するには、FlexCharのSymbolRenderingイベントを使用し、手動で合計値を計算してDrawStringメソッドで描画します。

◎サンプルコード（VB）

Imports C1.Chart
Imports C1.Win.Chart

Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        'データテーブルの設定
        Dim table1 As DataTable = New DataTable()
        table1.Columns.Add(New DataColumn("ID", GetType(System.Double)))
        table1.Columns.Add(New DataColumn("個数", GetType(System.Double)))
        table1.Rows.Add(New Object() {1, 10})
        table1.Rows.Add(New Object() {2, 30})
        table1.Rows.Add(New Object() {3, 20})
        table1.Rows.Add(New Object() {4, 30})
        table1.Rows.Add(New Object() {5, 10})
        table1.AcceptChanges()
        Dim table2 As DataTable = New DataTable()
        table2.Columns.Add(New DataColumn("ID", GetType(System.Double)))
        table2.Columns.Add(New DataColumn("個数", GetType(System.Double)))
        table2.Rows.Add(New Object() {1, 30})
        table2.Rows.Add(New Object() {2, 20})
        table2.Rows.Add(New Object() {3, 10})
        table2.Rows.Add(New Object() {4, 30})
        table2.Rows.Add(New Object() {5, 20})
        table2.AcceptChanges()

        'シリーズを追加
        FlexChart1.Series.Clear()
        Dim series1 As Series = New Series()
        series1.ChartType = ChartType.Column
        series1.DataSource = table1
        series1.BindingX = "ID"
        series1.Binding = "個数"
        FlexChart1.Series.Add(series1)
        Dim series2 As Series = New Series()
        series2.ChartType = ChartType.Column
        series2.DataSource = table2
        series2.BindingX = "ID"
        series2.Binding = "個数"
        FlexChart1.Series.Add(series2)

        '積層グラフの設定
        FlexChart1.Stacking = Stacking.Stacked

        'Y軸範囲の設定
        FlexChart1.AxisY.Min = 0
        FlexChart1.AxisY.Max = 70
    End Sub

    Private Sub FlexChart1_Rendering(sender As Object, e As RenderEventArgs) Handles FlexChart1.Rendering
        'データラベルの設定
        FlexChart1.DataLabel.Position = LabelPosition.Center
        FlexChart1.DataLabel.Content = "{y:#,##0}"

        '合計の算出 
        Dim s0 As Series = FlexChart1.Series(0)
        Dim s1 As Series = FlexChart1.Series(1)
        Dim val0() As Double = s0.GetValues(0)
        Dim val1() As Double = s1.GetValues(0)
        Dim valSum() As Double = New Double((val1.Length) - 1) {}
        Dim i As Integer
        For i = 0 To val1.Length - 1
            valSum(i) = (val0(i) + val1(i))
        Next

        '合計の表示
        Dim sumFont As Font = New Font(Me.Font, FontStyle.Bold)
        AddHandler s1.SymbolRendering,
            Sub(sndr, args)
                Dim offsetX As Integer = -10
                Dim offsetY As Integer = -20
                Dim y As Integer = CType(FlexChart1.AxisY.Convert(valSum(args.Index)), Integer)
                e.Graphics.DrawString($"{valSum(args.Index)}", sumFont, Brushes.Orange, New PointF((args.Point.X + offsetX), (y + offsetY)))
            End Sub
    End Sub
End Class

◎サンプルコード（C#）

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using C1.Chart;
using C1.Win.Chart;

namespace _20210224h
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            //データテーブルの設定
            var table1 = new DataTable();
            table1.Columns.Add(new DataColumn("ID", typeof(double)));
            table1.Columns.Add(new DataColumn("個数", typeof(double)));
            table1.Rows.Add(new object[] { 1, 10 });
            table1.Rows.Add(new object[] { 2, 30 });
            table1.Rows.Add(new object[] { 3, 20 });
            table1.Rows.Add(new object[] { 4, 30 });
            table1.Rows.Add(new object[] { 5, 10 });

            var table2 = new DataTable();
            table2.Columns.Add(new DataColumn("ID", typeof(double)));
            table2.Columns.Add(new DataColumn("個数", typeof(double)));
            table2.Rows.Add(new object[] { 1, 30 });
            table2.Rows.Add(new object[] { 2, 20 });
            table2.Rows.Add(new object[] { 3, 10 });
            table2.Rows.Add(new object[] { 4, 30 });
            table2.Rows.Add(new object[] { 5, 20 });

            //シリーズを追加
            flexChart1.Series.Clear();
            Series series1 = new Series();
            series1.ChartType = ChartType.Column;
            series1.DataSource = table1;
            series1.BindingX = "ID";
            series1.Binding = "個数";

            flexChart1.Series.Add(series1);
            Series series2 = new Series();
            series2.ChartType = ChartType.Column;
            series2.DataSource = table2;
            series2.BindingX = "ID";
            series2.Binding = "個数";
            flexChart1.Series.Add(series2);

            // 積層グラフの設定
            flexChart1.Stacking = Stacking.Stacked;

            // Y軸範囲の設定
            flexChart1.AxisY.Min = 0;
            flexChart1.AxisY.Max = 70;
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void flexChart1_Rendering(object sender, RenderEventArgs e)
        {
            // データラベルの設定
            flexChart1.DataLabel.Position = LabelPosition.Center;
            flexChart1.DataLabel.Content = "{y:#,##0}";

            // 合計の算出 
            Series s0 = flexChart1.Series[0];
            Series s1 = flexChart1.Series[1];
            double[] val0 = s0.GetValues(0);
            double[] val1 = s1.GetValues(0);
            double[] valSum = new double[val1.Length];
            for (var i = 0; i < val1.Length; i++)
            {
                valSum[i] = val0[i] + val1[i];
            }

            // 合計の表示
            Font sumFont = new Font(this.Font, FontStyle.Bold);
            s1.SymbolRendering += (sndr, args) =>
            {
                int offsetX = -10;
                int offsetY = -20;
                int y = (int)flexChart1.AxisY.Convert(valSum[args.Index]);
                e.Graphics.DrawString($"{valSum[args.Index]}", sumFont, Brushes.Orange, new PointF(args.Point.X + offsetX, y + offsetY));
            };
        }
    }
}