[Browsable(false)]
public CustomAggregateFunction CustomAggregateFunction { get; set; }

<Browsable(False)>
Public Property CustomAggregateFunction As CustomAggregateFunction

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TimeSpanExample"
        xmlns:dxc="http://schemas.devexpress.com/winfx/2008/xaml/charts" 
        x:Class="TimeSpanExample.MainWindow"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.DataContext>
        <local:ChartViewModel/>
    </Window.DataContext>
    <Grid>
        <dxc:ChartControl>
            <dxc:XYDiagram2D>
                <!-- Series settings are skipped. -->
                <dxc:XYDiagram2D.AxisX>
                    <dxc:AxisX2D>
                        <dxc:AxisX2D.TimeSpanScaleOptions>
                            <dxc:ManualTimeSpanScaleOptions AggregateFunction="Custom">
                                <dxc:ManualTimeSpanScaleOptions.CustomAggregateFunction>
                                    <local:StandardDeviationAggregateFunction/>
                                </dxc:ManualTimeSpanScaleOptions.CustomAggregateFunction>
                            </dxc:ManualTimeSpanScaleOptions>
                        </dxc:AxisX2D.TimeSpanScaleOptions>
                    </dxc:AxisX2D>
                </dxc:XYDiagram2D.AxisX>
            </dxc:XYDiagram2D>
        </dxc:ChartControl>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using DevExpress.Xpf.Charts;

namespace TimeSpanExample {
    public class StandardDeviationAggregateFunction : CustomAggregateFunction {
        public override string ToString() {
            return "StdDev (Custom)";
        }
        public override double[] Calculate(GroupInfo groupInfo) {
            double sum = 0.0;
            foreach (double value in groupInfo.Values1) {
                sum += value;
            }
            int len = groupInfo.Values1.Count();
            double averageAmount = sum / len;
            double standardDeviationSquareSum = 0.0;
            foreach (double value in groupInfo.Values1) {
                double deviation = value - averageAmount;
                standardDeviationSquareSum += deviation * deviation;
            }
            double stdDev = Math.Sqrt(standardDeviationSquareSum / len);
            return new double[1] { stdDev };
        }
    }
}

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Windows
Imports DevExpress.Xpf.Charts

Namespace TimeSpanExample
    Public Class StandardDeviationAggregateFunction
        Inherits CustomAggregateFunction

        Public Overrides Function ToString() As String
            Return "StdDev (Custom)"
        End Function

        Public Overrides Function Calculate(ByVal groupInfo As GroupInfo) As Double()
            Dim sum As Double = 0.0

            For Each value As Double In groupInfo.Values1
                sum += value
            Next

            Dim len As Integer = groupInfo.Values1.Count()
            Dim averageAmount As Double = sum / len
            Dim standardDeviationSquareSum As Double = 0.0

            For Each value As Double In groupInfo.Values1
                Dim deviation As Double = value - averageAmount
                standardDeviationSquareSum += deviation * deviation
            Next

            Dim stdDev As Double = Math.Sqrt(standardDeviationSquareSum / len)
            Return New Double(0) {stdDev}
        End Function
    End Class
End Namespace