WPF animating a StackPanel's width from 0 to Auto?

I am trying to animate a StackPanel when its visibility changed to grow from a width of 0 to its automatic width, here is what I have at the moment:

<Trigger Property="Visibility" Value="Visible">
    <Setter Property="Width" Value="0"></Setter>
    <Trigger.EnterActions>
        <BeginStoryboard>
            <Storyboard>
                <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Width" Duration="0:0:1">
                    <DiscreteObjectKeyFrame KeyTime="0">
                        <DiscreteObjectKeyFrame.Value>
                            <System:Double>NaN</System:Double>
                        </DiscreteObjectKeyFrame.Value>
                    </DiscreteObjectKeyFrame>
                </ObjectAnimationUsingKeyFrames>
            </Storyboard>
        </BeginStoryboard>
    </Trigger.EnterActions>
</Trigger>

Can someone explain how I might achieve this animation? Is it maybe not possible in the way I am trying to do it?

Thanks, alex.

Solution

Here is a quick mockup project I threw together.

In the Window's Loaded event, I simply set the stackpanel's visibility to Visible and it expands to fit its container width from left to right... Hopefully that's suits your needs.

Some things to note:

You must predefine the scale transform, else the animation will not play.
If you omit To in an animation, it will animate back to the default value.

And here is the code:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="600" Loaded="Window_Loaded">
    <Border HorizontalAlignment="Center" Width="300" Background="Gainsboro">
        <Border.Resources>
            <Style TargetType="StackPanel" x:Key="expand">
                <Setter Property="RenderTransform">
                    <Setter.Value>
                        <ScaleTransform ScaleX="1"/>
                    </Setter.Value>
                </Setter>
                <Style.Triggers>
                    <Trigger Property="Visibility" Value="Visible">
                        <Trigger.EnterActions>
                            <BeginStoryboard>
                                <Storyboard>
                                    <DoubleAnimation Storyboard.TargetProperty="RenderTransform.ScaleX"
                                                     From="0"
                                                     Duration="0:00:01"/>
                                </Storyboard>
                            </BeginStoryboard>
                        </Trigger.EnterActions>
                    </Trigger>
                </Style.Triggers>
            </Style>
        </Border.Resources>

        <StackPanel x:Name="stackpanel" Background="Gray" Visibility="Collapsed" Style="{StaticResource expand}"/>

    </Border>
</Window>