Dim maxdate As Date = _
            Aggregate drow As DataRow In dtb.AsEnumerable _
            Into Max(drow.Field(Of Date)("Date"))
 
    Dim mindate As Date = _
            Aggregate drow As DataRow In dtb.AsEnumerable _
            Into Min(drow.Field(Of Date)("Date"))
 
    Const limit As Double = 35.9
 
    cht.Series.Add("Limit")
    With cht.Series("Limit")
       .ChartType = DataVisualization.Charting.SeriesChartType.Line
       .Points.AddXY(mindate, limit)
       .Points.AddXY(maxdate, limit)
       .Color = Color.Blue
    End With