One way of approaching this would be to basically just hi-jack the zoom change and use it to determine
selected axis range. You would, of course, need to be craftier if you wanted to be able to zoom or select.
Let me know if this helps!

The xaml:

<UserControl.Resources>
        <local:TestData x:Key="data1" />
    </UserControl.Resources>


        <Grid x:Name="LayoutRoot" Background="White">
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

            <ig:XamDataChart Name="xamDataChart1" 
                         VerticalZoomable="True"
                         HorizontalZoomable="True"
                         >
            <ig:XamDataChart.Axes>
                <ig:CategoryXAxis x:Name="xAxis" 
                                  ItemsSource="{StaticResource data1}" 
                                  Label="{}{Label}"/>
                <ig:NumericYAxis x:Name="yAxis" />
            </ig:XamDataChart.Axes>
            <ig:XamDataChart.Series>
                <ig:LineSeries x:Name="testLine" 
                               ItemsSource="{StaticResource data1}" 
                               XAxis="{Binding ElementName=xAxis}"
                               YAxis="{Binding ElementName=yAxis}"
                               ValueMemberPath="Value"
                               />
            </ig:XamDataChart.Series>
        </ig:XamDataChart>

        <StackPanel Grid.Row="1" Orientation="Horizontal" >
            <TextBlock Text="Selected: " />
            <TextBlock Text="X: " />
            <TextBlock Text="{Binding StartSelectionX}" />
            <TextBlock Text="," />
            <TextBlock Text="{Binding EndSelectionX}" />
            <TextBlock Text="Y:" />
            <TextBlock Text="{Binding StartSelectionY}" />
            <TextBlock Text="," />
            <TextBlock Text="{Binding EndSelectionY}" />
        </StackPanel>
    </Grid>

And the code behind:

public partial class MainPage : UserControl, INotifyPropertyChanged
    {
        public MainPage()
        {
            InitializeComponent();

            DataContext = this;

            SetBinding(
                AttemptedScaleProperty,
                new Binding("WindowRect") { Source = xamDataChart1 });
        }

        public static readonly DependencyProperty AttemptedScaleProperty
            = DependencyProperty.Register(
            "AttemptedScale",
            typeof(Rect),
            typeof(MainPage),
            new PropertyMetadata(
                Rect.Empty,
                (o, e) => (o as MainPage).OnAttemptedScaleChanged(e)));

        public Rect AttemptedScale
        {
            get { return (Rect)GetValue(AttemptedScaleProperty); }
            set { SetValue(AttemptedScaleProperty, value); }
        }

        private Rect _selectedRect;
        private bool _recursing;

        private double _startSelectionX;
        public double StartSelectionX
        {
            get { return _startSelectionX; }
            set
            {
                _startSelectionX = value;
                if (PropertyChanged != null)
                {
                    PropertyChanged(
                        this, 
                        new PropertyChangedEventArgs("StartSelectionX"));
                }
            }
        }

        private double _endSelectionX;
        public double EndSelectionX
        {
            get { return _endSelectionX; }
            set
            {
                _endSelectionX = value;
                if (PropertyChanged != null)
                {
                    PropertyChanged(
                        this,
                        new PropertyChangedEventArgs("EndSelectionX"));
                }
            }
        }

        private double _startSelectionY;
        public double StartSelectionY
        {
            get { return _startSelectionY; }
            set
            {
                _startSelectionY = value;
                if (PropertyChanged != null)
                {
                    PropertyChanged(
                        this,
                        new PropertyChangedEventArgs("StartSelectionY"));
                }
            }
        }

        private double _endSelectionY;
        public double EndSelectionY
        {
            get { return _endSelectionY; }
            set
            {
                _endSelectionY = value;
                if (PropertyChanged != null)
                {
                    PropertyChanged(
                        this,
                        new PropertyChangedEventArgs("EndSelectionY"));
                }
            }
        }

        private void OnAttemptedScaleChanged(
            DependencyPropertyChangedEventArgs e)
        {
            if ((Rect)e.NewValue != _selectedRect &&
                (Rect)e.OldValue != Rect.Empty &&
                !_recursing)
            {
                _selectedRect = (Rect)e.NewValue;
                _recursing = true;
                xamDataChart1.WindowRect = (Rect)e.OldValue;
                _recursing = false;

                Rect viewportRect = new Rect(0,0,1,1);
                CategoryXAxis x = xamDataChart1.Axes[0] as CategoryXAxis;
                NumericYAxis y = xamDataChart1.Axes[1] as NumericYAxis;

                StartSelectionX = 
                    x.GetUnscaledValue(0, _selectedRect, viewportRect);
                EndSelectionX = 
                    x.GetUnscaledValue(1, _selectedRect, viewportRect);

                StartSelectionY = 
                    y.GetUnscaledValue(0, _selectedRect, viewportRect);
                EndSelectionY = 
                    y.GetUnscaledValue(1, _selectedRect, viewportRect);
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }


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

    public class TestData
        : ObservableCollection<TestDataItem>
    {
        private static Random rand = new Random();

        public TestData()
        {
            double curr = 0;
            for (int i = 0; i < 1000; i++)
            {
                if (rand.NextDouble() > .5)
                {
                    curr += rand.NextDouble();
                }
                else
                {
                    curr -= rand.NextDouble();
                }

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

-Graham