Hello Chen Alice,

Seeing your screenshot, it seems that you are using Crosshair, is this correct?

In that case, you can get the Crosshair point from xamDataChart's CrosshairPoint property. And then, get the series object you want to display the values from chart's Series property and get the data item nearest to the Crosshair point by calling series' GetItem method. The event to do this imelementation would be chart's MouseMove event.

private void xamDataChart1_MouseMove(object sender, MouseEventArgs e)
{
    // Get the sender chart instant
    XamDataChart chart = sender as XamDataChart;

    // Get the world coordinate of the Crosshair
    Point crosshairPoint = chart.CrosshairPoint;
    if (Double.IsNaN(crosshairPoint.X) || Double.IsNaN(crosshairPoint.Y))
    {
        return;
    }

    // Get the "lineSeries1"
    Series series = chart.Series.FirstOrDefault(s => s.Name == "lineSeries1");
    if (series == null)
    {
        return;
    }

    // Get the data item nearest to the Crosshair
    DataPoint dataPoint = series.GetItem(crosshairPoint) as DataPoint;  // Cast to the class of your data model.
    // If we get it
    if (dataPoint != null)
    {
        // Update the textboxes with the data item values.
        // NOTE: Format the values by yourself if necessary.
        textBlockLine1X.Text = dataPoint.XValue.ToString();
        textBlockLine1Y.Text = dataPoint.YValue.ToString();
    }
}

I attached a sample which use the above codes for your reference.

3527.F121987-SampleApp1.zip