Here's an example of animating the layout size of the chart vs. animating the render size of the chart.
Layout is the top chart, Render the bottom.

Here is the Xaml:

<Grid Height="800">
        <Grid.RowDefinitions>
            <RowDefinition Height="400" />
            <RowDefinition Height="400" />
        </Grid.RowDefinitions>
        
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        
        <Grid x:Name="layout" Grid.Row="0">
            <igChart:XamChart x:Name="chart1" Width="400" Height="400" >
                <igChart:XamChart.Axes>
                    <igChart:Axis AxisType="PrimaryX" Unit="1000" />
                </igChart:XamChart.Axes>
                <igChart:XamChart.Series>
                    <igChart:Series 
                        ChartType="Line"
                        DataMapping="Label = Label; Value = Value"
                        DataSource="{Binding}" />
                </igChart:XamChart.Series>
            </igChart:XamChart>
        </Grid>
        
        <Viewbox x:Name="render" Grid.Row="1"
                 Stretch="Uniform">
            <igChart:XamChart x:Name="chart2" Width="400" Height="400" >
                <igChart:XamChart.Axes>
                    <igChart:Axis AxisType="PrimaryX" Unit="1000" />
                </igChart:XamChart.Axes>
                <igChart:XamChart.Series>
                    <igChart:Series 
                        ChartType="Line"
                        DataMapping="Label = Label; Value = Value"
                        DataSource="{Binding}" />
                </igChart:XamChart.Series>
            </igChart:XamChart>
        </Viewbox>

        <Button x:Name="resize1" Grid.Row="0" Grid.Column="1" Content="Resize" >
            <Button.Triggers>
                <EventTrigger RoutedEvent="Button.Click">
                    <BeginStoryboard>
                        <Storyboard>
                            <DoubleAnimation 
                                From="{Binding ElementName=chart1, Path=ActualWidth}"
                                To="0" 
                                Duration="0:00:01"
                                Storyboard.Target="{Binding ElementName=chart1}" 
                                Storyboard.TargetProperty="Width" />
                            <DoubleAnimation 
                                From="{Binding ElementName=chart1, Path=ActualHeight}"
                                To="0" 
                                Duration="0:00:01"
                                Storyboard.Target="{Binding ElementName=chart1}" 
                                Storyboard.TargetProperty="Height" />
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </Button.Triggers>
        </Button>
        <Button x:Name="resize2" Grid.Row="1" Grid.Column="1" Content="Resize" >
            <Button.Triggers>
                <EventTrigger RoutedEvent="Button.Click">
                    <BeginStoryboard>
                        <Storyboard>
                            <DoubleAnimation 
                                From="{Binding ElementName=render, Path=ActualWidth}"
                                To="0" 
                                Duration="0:00:01"
                                Storyboard.Target="{Binding ElementName=render}" 
                                Storyboard.TargetProperty="Width" />
                            <DoubleAnimation 
                                From="{Binding ElementName=render, Path=ActualHeight}"
                                To="0" 
                                Duration="0:00:01"
                                Storyboard.Target="{Binding ElementName=render}" 
                                Storyboard.TargetProperty="Height" />
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </Button.Triggers>
        </Button>
    </Grid>

And here is the code behind:

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            DataContext = new TestData();
        }
    }

    public class TestDataItem
    {
        public string Label { get; set; }
        public double Value { get; set; }
    }

    public class TestData
        : ObservableCollection<TestDataItem>
    {
        public TestData()
        {
            Random rand = new Random();
            double curr = 5.0;
            for (int i = 0; i < 10000; i++)
            {
                if (rand.NextDouble() > .5)
                {
                    curr += rand.NextDouble();
                }
                else
                {
                    curr -= rand.NextDouble();
                }

                Add(new TestDataItem()
                {
                    Label = i.ToString(),
                    Value = curr
                });
            }
        }
    }

Hope this helps!

-Graham