Here's one way of going about it:

<UserControl.Resources>
        <Style x:Key="popupLegend" TargetType="igChart:XamWebChart">
            <Style.Setters>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="igChart:XamWebChart">
                            <Border BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}">
                                <Grid x:Name="RootElement" 
                                      Background="{TemplateBinding Background}" 
                                      Margin="{TemplateBinding Padding}" >
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="*"/>
                                        <!--<ColumnDefinition Width="Auto" MaxWidth="200"/>-->
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="Auto"/>
                                        <RowDefinition Height="*"/>
                                    </Grid.RowDefinitions>
                                    <Grid x:Name="CaptionPanel" Grid.Row="0" Grid.Column="0"/>
                                    <Grid x:Name="ScenePanel" Grid.Column="0" Grid.Row="1"/>
                                    <Grid x:Name="LegendPanel" Opacity="0" Margin="0,20,0,0" MaxWidth="200" Grid.Row="1" 
                                          VerticalAlignment="Top" HorizontalAlignment="Right"/>
                                    <local:RolloverPanel HorizontalAlignment="Right" Grid.Row="1" VerticalAlignment="Top" >
                                        <local:RolloverPanel.MouseEnterStoryBoard>
                                            <Storyboard>
                                                <DoubleAnimation
                                                    Storyboard.TargetName="LegendPanel"
                                                    Storyboard.TargetProperty="Opacity"
                                                    To="1.0"
                                                    Duration="0:0:0.2" />
                                            </Storyboard>
                                        </local:RolloverPanel.MouseEnterStoryBoard>
                                        <local:RolloverPanel.MouseLeaveStoryBoard>
                                            <Storyboard>
                                                <DoubleAnimation
                                                    Storyboard.TargetName="LegendPanel"
                                                    Storyboard.TargetProperty="Opacity"
                                                    To="0.0"
                                                    Duration="0:0:0.2" />
                                            </Storyboard>
                                        </local:RolloverPanel.MouseLeaveStoryBoard>
                                        <TextBlock x:Name="legendText" Opacity=".5" Text="* Legend" Height="20" />
                                    </local:RolloverPanel>
                                </Grid>
                            </Border>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style.Setters>
        </Style>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="White">
        <igChart:XamWebChart x:Name="theChart" Style="{StaticResource popupLegend}" >
            <igChart:XamWebChart.Legend>
                <igChart:Legend Width="100" BorderBrush="Black" BorderThickness="1"/>
            </igChart:XamWebChart.Legend>

            <igChart:XamWebChart.Series>
                <igChart:Series ChartType="Column" >
                    <igChart:Series.DataPoints>
                        <igChart:DataPoint Label="A" Value="1" />
                        <igChart:DataPoint Label="B" Value="2" />
                        <igChart:DataPoint Label="C" Value="3" />
                    </igChart:Series.DataPoints>
                </igChart:Series>
            </igChart:XamWebChart.Series>
        </igChart:XamWebChart>
    </Grid>

With code behind:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }
    }

    public class RolloverPanel
        : ContentControl
    {
        public static readonly DependencyProperty MouseEnterStoryboardProperty =
            DependencyProperty.Register(
            "MouseEnterStoryboard",
            typeof(Storyboard),
            typeof(RolloverPanel),
            new PropertyMetadata(null));

        public Storyboard MouseEnterStoryBoard
        {
            get { return (Storyboard)GetValue(MouseEnterStoryboardProperty); }
            set { SetValue(MouseEnterStoryboardProperty, value); }
        }

        public static readonly DependencyProperty MouseLeaveStoryboardProperty =
            DependencyProperty.Register(
            "MouseLeaveStoryboard",
            typeof(Storyboard),
            typeof(RolloverPanel),
            new PropertyMetadata(null));

        public Storyboard MouseLeaveStoryBoard
        {
            get { return (Storyboard)GetValue(MouseLeaveStoryboardProperty); }
            set { SetValue(MouseLeaveStoryboardProperty, value); }
        }

        public RolloverPanel()
        {
            MouseEnter += RolloverPanel_MouseEnter;
            MouseLeave += RolloverPanel_MouseLeave;
        }

        void RolloverPanel_MouseLeave(object sender, MouseEventArgs e)
        {
            if (MouseLeaveStoryBoard == null)
            {
                return;
            }
            MouseLeaveStoryBoard.Begin();
        }

        void RolloverPanel_MouseEnter(object sender, MouseEventArgs e)
        {
            if (MouseEnterStoryBoard == null)
            {
                return;
            }
            MouseEnterStoryBoard.Begin();
        }
    }

-Graham