using System;
using System.Windows.Forms;
using DevExpress.XtraCharts;
// ...

const int interval = 20;
Random random = new Random();
int TimeInterval = 10;
double value1 = 10.0;

AxisRange AxisXRange {
    get {
        SwiftPlotDiagram diagram = chartControl1.Diagram as SwiftPlotDiagram;
        if (diagram != null)
            return diagram.AxisX.Range;
        return null;
    }
}

double CalculateNextValue(double value) {
    return value + (random.NextDouble() * 10.0 - 5.0);
}

void UpdateValues() {
    value1 = CalculateNextValue(value1);
}

private void timer1_Tick(object sender, EventArgs e) {
    Series series1 = chartControl1.Series[0];

    if (series1 == null)
        return;
    DateTime argument = DateTime.Now;
    SeriesPoint[] pointsToUpdate1 = new SeriesPoint[interval];
    for (int i = 0; i < interval; i++) {
        pointsToUpdate1[i] = new SeriesPoint(argument, value1);
        argument = argument.AddMilliseconds(1);
        UpdateValues();
    }
    DateTime minDate = argument.AddSeconds(-TimeInterval);
    int pointsToRemoveCount = 0;
    foreach (SeriesPoint point in series1.Points)
        if (point.DateTimeArgument < minDate)
            pointsToRemoveCount++;
    if (pointsToRemoveCount < series1.Points.Count)
        pointsToRemoveCount--;
    series1.Points.AddRange(pointsToUpdate1);
    if (pointsToRemoveCount > 0) {
        series1.Points.RemoveRange(0, pointsToRemoveCount);
    }
    if (AxisXRange != null) {
        AxisXRange.SetMinMaxValues(minDate, argument);
    }
}

Imports System
Imports System.Windows.Forms
Imports DevExpress.XtraCharts
' ...

Private Const interval As Integer = 20
    Private random As New Random()
    Private TimeInterval As Integer = 10
    Private value1 As Double = 10.0

    Private ReadOnly Property AxisXRange() As AxisRange
    Get
        Dim diagram As SwiftPlotDiagram = TryCast(chartControl1.Diagram, SwiftPlotDiagram)
        If diagram IsNot Nothing Then
            Return diagram.AxisX.Range
        End If
        Return Nothing
    End Get
End Property

Private Function CalculateNextValue(ByVal value As Double) As Double
    Return value + (random.NextDouble() * 10.0 - 5.0)
End Function

Private Sub UpdateValues()
    value1 = CalculateNextValue(value1)
End Sub

Private Sub timer1_Tick(ByVal sender As Object, ByVal e As EventArgs) Handles timer1.Tick
    Dim series1 As Series = chartControl1.Series(0)

    If series1 Is Nothing Then
        Return
    End If
    Dim argument As DateTime = DateTime.Now
    Dim pointsToUpdate1(interval - 1) As SeriesPoint
    For i As Integer = 0 To interval - 1
        pointsToUpdate1(i) = New SeriesPoint(argument, value1)
        argument = argument.AddMilliseconds(1)
        UpdateValues()
    Next i
    Dim minDate As DateTime = argument.AddSeconds(-TimeInterval)
    Dim pointsToRemoveCount As Integer = 0
    For Each point As SeriesPoint In series1.Points
        If point.DateTimeArgument < minDate Then
            pointsToRemoveCount += 1
        End If
    Next point
    If pointsToRemoveCount < series1.Points.Count Then
        pointsToRemoveCount -= 1
    End If
    series1.Points.AddRange(pointsToUpdate1)
    If pointsToRemoveCount > 0 Then
        series1.Points.RemoveRange(0, pointsToRemoveCount)
    End If
    If AxisXRange IsNot Nothing Then
        AxisXRange.SetMinMaxValues(minDate, argument)
    End If
End Sub