在datagrid columnheader中绘制线条时绑定不起作用

Question

我想使用以下代码在数据网格的列标题中绘制线条。有两个ItemsControl都绑定到包含18个日期的集合（每个月的第一个）。转换器按日期计算列标题中的位置，以便将控件放在列标题中。

<DataGridTemplateColumn Width="1*" x:Name="coPlanung">

    <DataGridTemplateColumn.HeaderTemplate>
        <DataTemplate>
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition />
                    <RowDefinition />
                </Grid.RowDefinitions>
                <!--Monatsnamen-->
                <ItemsControl Grid.Row="0" ItemsSource="{Binding DataContext.Monate, ElementName=dgBaustellen}" Margin="0" VerticalAlignment="Center" HorizontalAlignment="Left" Background="LightBlue">
                    <ItemsControl.ItemContainerStyle>
                        <Style TargetType="ContentPresenter">
                            <Setter Property="Canvas.Left">
                                <Setter.Value>
                                    <MultiBinding Converter="{StaticResource d2x}">
                                        <Binding Path="Date" Mode="OneWay" />
                                        <Binding Path="ActualWidth" ElementName="coPlanung" Mode="OneWay"/>
                                    </MultiBinding>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </ItemsControl.ItemContainerStyle>
                    <ItemsControl.ItemsPanel>
                        <ItemsPanelTemplate>
                            <Canvas Width="{Binding ActualWidth, ElementName=coPlanung}" Margin="-5,0,0,0" Background="LemonChiffon"/>
                        </ItemsPanelTemplate>
                    </ItemsControl.ItemsPanel>
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>
                            <!--Monatsnamen-->
                            <Border BorderThickness="1,0,1,0" BorderBrush="Black">
                                <TextBox Text="{Binding Path=Date, StringFormat={}{0:MMM-yyyy}, Mode=OneWay}" 
                                         HorizontalAlignment="Center" VerticalAlignment="Center" HorizontalContentAlignment="Center"
                                         Background="Transparent" Margin="0" BorderThickness="0">
                                    <TextBox.Width>
                                        <MultiBinding Converter="{StaticResource d2b}">
                                            <Binding Path="Date" Mode="OneWay" />
                                            <Binding Path="ActualWidth" ElementName="coPlanung" Mode="OneWay"/>
                                        </MultiBinding>
                                    </TextBox.Width>
                                </TextBox>
                            </Border>
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                </ItemsControl>
                <!--Linien-->
                <ItemsControl Grid.Row="1" ItemsSource="{Binding DataContext.Monate, ElementName=dgBaustellen}" >
                    <ItemsControl.ItemContainerStyle>
                        <Style TargetType="ContentPresenter">
                            <Setter Property="Canvas.Left">
                                <Setter.Value>
                                    <MultiBinding Converter="{StaticResource d2x}">
                                        <Binding Path="Date" Mode="OneWay" />
                                        <Binding Path="ActualWidth" ElementName="coPlanung" Mode="OneWay"/>
                                    </MultiBinding>
                                </Setter.Value>
                            </Setter>
                        </Style>
                    </ItemsControl.ItemContainerStyle>
                    <ItemsControl.ItemsPanel>
                        <ItemsPanelTemplate>
                            <Canvas Width="{Binding ActualWidth, ElementName=coPlanung}" Margin="-5,0,0,0" Background="LemonChiffon"/>
                        </ItemsPanelTemplate>
                    </ItemsControl.ItemsPanel>
                    <ItemsControl.ItemTemplate>
                        <DataTemplate>

                <!--Linien-->

                            <Line Y1="-10" Y2="30" Stroke="Red" >
                                <Line.X1>
                                    <MultiBinding Converter="{StaticResource d2x}" >
                                        <Binding Path="Date" Mode="OneWay" />
                                        <Binding Path="ActualWidth" ElementName="coPlanung" Mode="OneWay"/>
                                    </MultiBinding>
                                </Line.X1>
                                <Line.X2>
                                    <MultiBinding Converter="{StaticResource d2x}">
                                        <Binding Path="Date" Mode="OneWay" />
                                        <Binding Path="ActualWidth" ElementName="coPlanung" Mode="OneWay"/>
                                    </MultiBinding>
                                </Line.X2>
                            </Line>
                        </DataTemplate>
                    </ItemsControl.ItemTemplate>
                </ItemsControl>
            </Grid>
        </DataTemplate>
    </DataGridTemplateColumn.HeaderTemplate>

结果如下：

显然，我想要的是在月份名称之间的黑线上画一条红线。看起来只有每一条线都被绘制出来。

有人有想法解决这个问题吗？