Make ListViewItems slide inside ListView

Question

As I add items to my ListView I want them to appear and slide until they get out of the bounds of the ListView.

Currently I am using a ThicknessAnimation on the Margin property of the ListViewItem's.

But when I add another item at the index 0 of the listview the margin of the old ListViewItem is relative to the new item, making it slide faster.

This is what I have so far:

<Window x:Class="SliderApplication.MainWindow"
        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:SliderApplication"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.DataContext>
        <local:MainWindowViewModel />
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="10*" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <ListView Grid.Row="0" ItemsSource="{Binding Items}">
            <ListView.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal" />
                </ItemsPanelTemplate>
            </ListView.ItemsPanel>
            <ListView.ItemContainerStyle>
                <Style TargetType="{x:Type ListViewItem}">
                    <Style.Triggers>
                        <EventTrigger RoutedEvent="Loaded">
                            <EventTrigger.Actions>
                                <BeginStoryboard>
                                    <Storyboard>
                                        <ThicknessAnimation Storyboard.TargetProperty="Margin" From="0,0" To="500,0" Duration="0:0:15" />
                                    </Storyboard>
                                </BeginStoryboard>
                            </EventTrigger.Actions>
                        </EventTrigger>
                    </Style.Triggers>
                </Style>
            </ListView.ItemContainerStyle>
        </ListView>
        <Button Grid.Row="1" Command="{Binding AddCommand}" />
    </Grid>
</Window>

What is the best way to achieve the desired effect?