Hi Steven,

The only way I've found to change the order of the Legend is to dig into the controls visual tree and grab the StackPanel that is used to contain each individual item.  From there you can rearrange the children as you like.  I used the following behavior for this: 

public class RearrangeLegendItems : Behavior<Legend>
{
    public Legend Legend { get { return this.AssociatedObject; } }

    protected override void OnAttached()
    {
        base.OnAttached();

        Legend.LayoutUpdated += new EventHandler(Legend_LayoutUpdated);
    }

    void Legend_LayoutUpdated(object sender, EventArgs e)
    {
        // Get the stack panel that contains the legend items.
        ScrollContentPresenter presenter = (ScrollContentPresenter)FindElementByName(Legend, "ScrollContentPresenter");
        StackPanel sPanel = (StackPanel)(presenter.Content as ContentPresenter).Content;

        // Rearrange the legend items here.
        UIElement firstElement = sPanel.Children[0];
        sPanel.Children.RemoveAt(0);
        sPanel.Children.Add(firstElement);

        // Unhook the event so we don't do this all the time.
        Legend.LayoutUpdated -= Legend_LayoutUpdated;
    }

    DependencyObject FindElementByName(DependencyObject parent, string name)
    {
        if ((parent as FrameworkElement).Name == name)
            return parent;

        int childCount = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < childCount; i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(parent, i);
            child = FindElementByName(child, name);
            if (child != null)
                return child;
        }

        return null;
    }
}

You can see in the LayoutUpdated event handler that I'm grabbing the StackPanel from the legend and then grabbing the first item in the children list.  I then remove it from the list and re-add it at the bottom.  You would then use the behavior like so in your XAML: 

<ig:Legend>
    <i:Interaction.Behaviors>
        <b:RearrangeLegendItems/>
    </i:Interaction.Behaviors>
</ig:Legend>

Let me know if you have any questions on this.