This sample should show you the pieces you can use to put this together:

The xaml:

 <UserControl.Resources>
        <local:TestData x:Key="data" />
    </UserControl.Resources>
    
    <Grid x:Name="LayoutRoot" Background="White">
        <igChart:XamDataChart x:Name="theChart" MouseLeftButtonDown="theChart_MouseLeftButtonDown">
            <igChart:XamDataChart.Axes>
                <igChart:NumericYAxis x:Name="yAxis" />
                <igChart:CategoryDateTimeXAxis 
                    x:Name="xAxis"
                    ItemsSource="{StaticResource data}"
                    DateTimeMemberPath="Date"
                    Label="{}{Date}"/>
            </igChart:XamDataChart.Axes>
            <igChart:XamDataChart.Series>
                <igChart:LineSeries 
                    x:Name="series"
                    ItemsSource="{StaticResource data}"
                    XAxis="{Binding ElementName=xAxis}"
                    YAxis="{Binding ElementName=yAxis}"
                    ValueMemberPath="Value" />
            </igChart:XamDataChart.Series>
        </igChart:XamDataChart>
    </Grid>

And the code behind:

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

        private void theChart_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            var s = theChart.Series.FirstOrDefault();
            if (s == null)
            {
                return;
            }

            var position = e.GetPosition(s);

            //we are only accounting for CategoryDateTimeXAxis in the following logic.
            if (!s.Chart.Axes.OfType<CategoryDateTimeXAxis>().Any())
            {
                return;
            }

            var x = s.Chart.Axes.OfType<CategoryDateTimeXAxis>().First();
            var y = s.Chart.Axes.OfType<NumericYAxis>().First();

            var viewport = new Rect(0, 0, x.ActualWidth, y.ActualHeight);
            var window = s.Chart.WindowRect;

            var unscaledX = x.GetUnscaledValue(position.X, window, viewport);
            var unscaledY = y.GetUnscaledValue(position.Y, window, viewport);

            MessageBox.Show("x: " + new DateTime((long)unscaledX) + ", y:" + unscaledY);
        }
    }

    public class TestData
        : ObservableCollection<TestDataItem>
    {
        public TestData()
        {
            Add(new TestDataItem()
            {
                Date = DateTime.Now.AddDays(-4),
                Value = 1
            });
            Add(new TestDataItem()
            {
                Date = DateTime.Now.AddDays(-3),
                Value = 2
            });
            Add(new TestDataItem()
            {
                Date = DateTime.Now.AddDays(-2),
                Value = 3
            });
            Add(new TestDataItem()
            {
                Date = DateTime.Now.AddDays(-1),
                Value = 4
            });
        }
    }

    public class TestDataItem
    {
        public DateTime Date { get; set; }
        public double Value { get; set; }
    }

Let me know if you have any questions. Hope this helps!

-Graham