傾向線（近似直線）を表すTrendLineのStyle.LinePatternを設定しても有効になりません。
例えばLinePatternをDashDotDotに設定しても、傾向線が直線で描画されます。

※2021J v2（4.5.20212.513）ではこの現象は発生しません。

この問題は2022J v1（4.5.20221.548）で修正されました。
※修正版を適用しない場合の回避方法は以下の通りです。

本来の傾向線を非表示にして、Renderedイベント内で独自に直線を描画します。
※FitType = C1.Chart.FitType.Linearの場合に限定

◎サンプルコード（VB）

Imports C1.Win.Chart
Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        'サンプルデータ
        Dim dataTable As System.Data.DataTable = New System.Data.DataTable
        dataTable.Columns.Add("POINT_X", GetType(System.Double))
        dataTable.Columns.Add("POINT_Y", GetType(System.Double))
        dataTable.Rows.Add(dataTable.NewRow)
        dataTable.Rows.Add(dataTable.NewRow)
        dataTable.Rows(0)("POINT_X") = 0
        dataTable.Rows(0)("POINT_Y") = 0
        dataTable.Rows(1)("POINT_X") = 1
        dataTable.Rows(1)("POINT_Y") = 1

        '散布図
        FlexChart1.ChartType = C1.Chart.ChartType.Scatter
        FlexChart1.AxisX.Min = 0
        FlexChart1.AxisX.Max = 1.2
        FlexChart1.AxisY.Min = 0
        FlexChart1.AxisY.Max = 1.2

        '系列追加
        FlexChart1.Series.Clear()
        Dim series As Series = New Series
        FlexChart1.Series.Add(series)
        series.DataSource = dataTable
        series.BindingX = "POINT_X"
        series.Binding = "POINT_Y"

        '近似直線追加
        Dim trendLine As TrendLine = New TrendLine()
        trendLine.Name = "Trendline"
        trendLine.DataSource = dataTable
        trendLine.BindingX = "POINT_X"
        trendLine.Binding = "POINT_Y"
        trendLine.FitType = C1.Chart.FitType.Linear
        FlexChart1.Series.Add(trendLine)
        trendLine.Style.StrokeWidth = 3
        trendLine.Style.StrokeColor = Color.Black

        'LinePatternの設定が有効にならない
        trendLine.Style.LinePattern = C1.Chart.LinePatternEnum.DashDotDot

        '【対応策】：近似直線の非表示
        trendLine.Visibility = C1.Chart.SeriesVisibility.Hidden
    End Sub

    Private Sub FlexChart1_Rendered(sender As Object, e As C1.Win.Chart.RenderEventArgs) Handles FlexChart1.Rendered
        '//【対応策】：近似直線の描画
        Dim trendLine As TrendLine = CType(FlexChart1.Series(1), C1.Win.Chart.TrendLine)
        If (trendLine.Coefficients.Length = 2) Then
            Dim p As Pen = New Pen(Color.Black)
            p.DashStyle = Drawing2D.DashStyle.DashDotDot
            p.Width = 3
            Dim x1 = CType(FlexChart1.AxisX.Convert(trendLine.GetValues(1)(0)), Integer)
            Dim x2 = CType(FlexChart1.AxisX.Convert(trendLine.GetValues(1)((trendLine.GetValues(1).Length - 1))), Integer)
            Dim y1 = CType(FlexChart1.AxisY.Convert(trendLine.GetValues(0)(0)), Integer)
            Dim y2 = CType(FlexChart1.AxisY.Convert(trendLine.GetValues(0)((trendLine.GetValues(0).Length - 1))), Integer)
            e.Graphics.DrawLine(p, New Point(x1, y1), New Point(x2, y2))
        End If
    End Sub
End Class

◎サンプルコード（C#）

using C1.Win.Chart;

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

            //サンプルデータ
            System.Data.DataTable dataTable = new System.Data.DataTable();
            dataTable.Columns.Add("POINT_X", typeof(double));
            dataTable.Columns.Add("POINT_Y", typeof(double));

            dataTable.Rows.Add(dataTable.NewRow());
            dataTable.Rows.Add(dataTable.NewRow());

            dataTable.Rows[0]["POINT_X"] = 0;
            dataTable.Rows[0]["POINT_Y"] = 0;
            dataTable.Rows[1]["POINT_X"] = 1;
            dataTable.Rows[1]["POINT_Y"] = 1;

            //散布図
            flexChart.ChartType = C1.Chart.ChartType.Scatter;
            flexChart.AxisX.Min = 0;
            flexChart.AxisX.Max = 1.2;
            flexChart.AxisY.Min = 0;
            flexChart.AxisY.Max = 1.2;

            //系列追加
            flexChart.Series.Clear();
            Series series = new Series();
            flexChart.Series.Add(series);
            series.DataSource = dataTable;
            series.BindingX = "POINT_X";
            series.Binding = "POINT_Y";

            //近似直線追加
            var trendLine = new TrendLine()
            {
                Name = "Trendline",
                DataSource = dataTable,
                BindingX = "POINT_X",
                Binding = "POINT_Y",
                FitType = C1.Chart.FitType.Linear,
            };
            flexChart.Series.Add(trendLine);
            trendLine.Style.StrokeWidth = 3;
            trendLine.Style.StrokeColor = Color.Black;

            //LinePatternの設定が有効にならない
            trendLine.Style.LinePattern = C1.Chart.LinePatternEnum.DashDotDot;

            //【対応策】：近似直線の非表示
            trendLine.Visibility = C1.Chart.SeriesVisibility.Hidden;
        }

        private void flexChart_Rendered(object sender, C1.Win.Chart.RenderEventArgs e)
        {
            //【対応策】：近似直線の描画
            var trendLine = flexChart.Series[1] as TrendLine;
            if (trendLine.Coefficients.Length == 2)
            {
                using (Pen p = new Pen(Color.Black) { DashStyle = System.Drawing.Drawing2D.DashStyle.DashDotDot, Width = 3 })
                {
                    var x1 = (int)flexChart.AxisX.Convert(trendLine.GetValues(1)[0]);
                    var x2 = (int)flexChart.AxisX.Convert(trendLine.GetValues(1)[trendLine.GetValues(1).Length - 1]);
                    var y1 = (int)flexChart.AxisY.Convert(trendLine.GetValues(0)[0]);
                    var y2 = (int)flexChart.AxisY.Convert(trendLine.GetValues(0)[trendLine.GetValues(0).Length - 1]);
                    e.Graphics.DrawLine(p, new Point(x1, y1), new Point(x2, y2));
                }
            }
        }
    }
}