<Page
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Page.Resources>
        <XmlDataProvider x:Key="DataL">
            <x:XData>
                <Art>
                    <Title>A</Title>
                    <Bars>
                        <Left V="1" />
                        <Left Val="2" />
                        <Left Val="3" />
                    </Bars>
                    <Title>B</Title>
                    <Bars>
                        <Left V="3" />
                        <Left Val="4" />
                        <Left Val="5" />
                    </Bars>
                </Art>
            </x:XData>
        </XmlDataProvider>
        <DataTemplate x:Key="DT">
                <ListBox 
                        ItemsSource="{Binding Source=Art, XPath=Bars}"
                        >
                    <ListBoxItem>
                        <TextBlock Text="{Binding XPath=Left/@Val, StringFormat=Line: {0}}" />
                    </ListBoxItem>
                </ListBox>
        </DataTemplate>
    </Page.Resources>
    <Grid>
        <StackPanel>
            <ListBox
               ItemsSource="{Binding Source={StaticResource DataL}, XPath=Art}"
               ItemTemplate="{StaticResource DT}"/>
        </StackPanel>
    </Grid>
</Page>