<CollectionViewSource x:Key="src" Source="{Binding Model.dataTable}">
            <CollectionViewSource.GroupDescriptions>
                <PropertyGroupDescription PropertyName="groupId" />
            </CollectionViewSource.GroupDescriptions>
        </CollectionViewSource>

  <Style x:Key="GroupHeaderStyle" TargetType="{x:Type GroupItem}">
            <Setter Property="Margin" Value="0,0,0,5"/>
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type GroupItem}">
                        <Expander IsExpanded="True" BorderBrush="Black" BorderThickness="0,0,0,1">
                            <Expander.Background>
                                <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
                                    <GradientStop Offset="0.0" Color="#FF85c1e6"></GradientStop>
                                    <GradientStop Offset="0.5" Color="#FF0187dc"></GradientStop>
                                    <GradientStop Offset="1.0" Color="#FF77c1e8"></GradientStop>
                                </LinearGradientBrush>
                            </Expander.Background>
                            <Expander.Header>
                                <DockPanel>
                                    <TextBlock FontWeight="Bold" Foreground="Black" Text="{Binding Source={StaticResource src}, Path=groupName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Margin="5,0,0,0" />
                                                                    </DockPanel>
                            </Expander.Header>
                            <Expander.Content>
                                <Border Background="White" Margin="2" CornerRadius="3">
                                    <ItemsPresenter />
                                </Border>
                            </Expander.Content>
                        </Expander>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>