本コンポーネントには、アラームゾーンを描画するための専用の機能は用意されていません。
代替策として、RenderedイベントとPaintイベントを利用し、独自に矩形を描画する方法が考えられます。

最初にRenderedイベントにおいて、指定した矩形範囲をコントロール座標に変換します（DataToPointメソッドを使用）。次いでPaintイベントにて、e.Graphics.FillRectangleメソッドを用いてこの矩形を塗りつぶします。

以下に、レベル＝0～20、20～80、80～100の３つのゾーンに分けて色分けするコードを記載します。



◎サンプルコード（VB）

Imports C1.Chart
Imports C1.Win.Chart

Public Class Form1
    Dim posA1, posA2, posB1, posB2, posC1, posC2 As Point
    Dim isFirstTime As Boolean = True

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        ' データの作成
        Dim dt As New DataTable()
        dt.Columns.AddRange({New DataColumn("時間", GetType(Double)), New DataColumn("レベル", GetType(Double))})
        Dim r As New Random()
        For i = 0 To 49
            dt.Rows.Add(i, r.Next(0, 100))
        Next
        dt.AcceptChanges()

        ' シリーズへのバインド
        Dim series1 As Series = New Series()
        series1.ChartType = ChartType.Line  'チャートタイプの設定
        series1.DataSource = dt
        series1.BindingX = "時間"
        series1.Binding = "レベル"
        FlexChart1.Series.Clear()
        FlexChart1.Series.Add(series1)
        FlexChart1.AxisX.Title = "時間"
        FlexChart1.AxisY.Title = "レベル"

        ' 軸の設定 
        FlexChart1.AxisX.Min = 0
        FlexChart1.AxisX.Max = 50
        FlexChart1.AxisY.Min = 0
        FlexChart1.AxisY.Max = 100
    End Sub

    Private Sub FlexChart1_Rendered(sender As Object, e As RenderEventArgs) Handles FlexChart1.Rendered
        If isFirstTime Then
            ' フラグの初期化
            isFirstTime = False
            ' 描画領域の取得
            posA1 = Point.Truncate(FlexChart1.DataToPoint(New PointF(0.0F, 100.0F)))
            posA2 = Point.Truncate(FlexChart1.DataToPoint(New PointF(50.0F, 80.0F)))
            posB1 = Point.Truncate(FlexChart1.DataToPoint(New PointF(0.0F, 80.0F)))
            posB2 = Point.Truncate(FlexChart1.DataToPoint(New PointF(50.0F, 20.0F)))
            posC1 = Point.Truncate(FlexChart1.DataToPoint(New PointF(0.0F, 20.0F)))
            posC2 = Point.Truncate(FlexChart1.DataToPoint(New PointF(50.0F, 0.0F)))
            ' 描画の強制
            FlexChart1.Invalidate()
        End If
    End Sub

    Private Sub FlexChart1_Paint(sender As Object, e As PaintEventArgs) Handles FlexChart1.Paint
        ' ゾーンのの塗りつぶし
        e.Graphics.FillRectangle(Brushes.Orange, New Rectangle(posA1, New Size(Math.Abs(posA2.X - posA1.X), Math.Abs(posA2.Y - posA1.Y))))
        e.Graphics.FillRectangle(Brushes.LightBlue, New Rectangle(posB1, New Size(Math.Abs(posB2.X - posB1.X), Math.Abs(posB2.Y - posB1.Y))))
        e.Graphics.FillRectangle(Brushes.Blue, New Rectangle(posC1, New Size(Math.Abs(posC2.X - posC1.X), Math.Abs(posC2.Y - posC1.Y))))
    End Sub
End Class

◎サンプルコード（C#）

using C1.Chart;
using C1.Win.Chart;

namespace prj_C1FlexChart
{
    public partial class Form1 : Form
    {
        Point posA1, posA2, posB1, posB2, posC1, posC2;
        bool isFirstTime = true;

        public Form1()
        {
            InitializeComponent();

            // データの作成
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[]
            {
                new DataColumn("時間", typeof(double)),
                new DataColumn("レベル", typeof(double))
            });
            Random r = new Random();
            for (var i = 0; i <= 49; i++)
                dt.Rows.Add(i, r.Next(0, 100));
            dt.AcceptChanges();

            // シリーズへのバインド
            flexChart1.Series.Clear();
            Series series1 = new Series();
            series1.ChartType = ChartType.Line;  // チャートタイプの設定
            series1.DataSource = dt;
            series1.BindingX = "時間";
            series1.Binding = "レベル";
            flexChart1.Series.Add(series1);
            flexChart1.AxisX.Title = "時間";
            flexChart1.AxisY.Title = "レベル";

            // 軸の設定 
            flexChart1.AxisX.Min = 0;
            flexChart1.AxisX.Max = 50;
            flexChart1.AxisY.Min = 0;
            flexChart1.AxisY.Max = 100;
        }

        private void flexChart1_Rendered(object sender, RenderEventArgs e)
        {
            if (isFirstTime)
            {
                // フラグの初期化
                isFirstTime = false;
                // 描画領域の取得
                posA1 = Point.Truncate(flexChart1.DataToPoint(new PointF(0.0F, 100.0F)));
                posA2 = Point.Truncate(flexChart1.DataToPoint(new PointF(50.0F, 80.0F)));
                posB1 = Point.Truncate(flexChart1.DataToPoint(new PointF(0.0F, 80.0F)));
                posB2 = Point.Truncate(flexChart1.DataToPoint(new PointF(50.0F, 20.0F)));
                posC1 = Point.Truncate(flexChart1.DataToPoint(new PointF(0.0F, 20.0F)));
                posC2 = Point.Truncate(flexChart1.DataToPoint(new PointF(50.0F, 0.0F)));
                // 描画の強制
                flexChart1.Invalidate();
            }
        }

        private void flexChart1_Paint(object sender, PaintEventArgs e)
        {
            // ゾーンのの塗りつぶし
            e.Graphics.FillRectangle(Brushes.Orange, new Rectangle(posA1, new Size(Math.Abs(posA2.X - posA1.X), Math.Abs(posA2.Y - posA1.Y))));
            e.Graphics.FillRectangle(Brushes.LightBlue, new Rectangle(posB1, new Size(Math.Abs(posB2.X - posB1.X), Math.Abs(posB2.Y - posB1.Y))));
            e.Graphics.FillRectangle(Brushes.Blue, new Rectangle(posC1, new Size(Math.Abs(posC2.X - posC1.X), Math.Abs(posC2.Y - posC1.Y))));
        }
    }
}