This sample illustrates the problem. When you run it., the window starts out at 300 x 300 and the values in the textblock are correct. If you maximize the Window and mouse over the chart, the values are incorrect.

<Window x:Class="HitTestTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ig="clr-namespace:Infragistics.Windows.Chart;assembly=Infragistics3.Wpf.Chart.v8.2"
    Title="Window1" Height="300" Width="300">
    <DockPanel>
        <TextBlock DockPanel.Dock="Bottom" x:Name="textBlock" TextAlignment="Center" />
        <ig:XamChart x:Name="xamChart" MouseMove="xamChart_MouseMove">
            <ig:XamChart.Series>
                <ig:Series x:Name="xamSeries" ChartType="ScatterLine" DataMapping="ValueX=X;ValueY=Y" />
            </ig:XamChart.Series>
        </ig:XamChart>
    </DockPanel>
</Window>

using System;
using System.Windows;
using System.Windows.Input;

namespace HitTestTest
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            this.Loaded += delegate
            {
                xamSeries.DataSource = new Point[]
                {
                    new Point { X = 0, Y = 0 },
                    new Point { X = 10, Y = 10},
                };
            };
        }

        private void xamChart_MouseMove(object sender, MouseEventArgs e)
        {
            var args = xamChart.HitTest(e);
            textBlock.Text = String.Format("({0:0.0},{1:0.0})",
                args.XValue, args.YValue);
        }
    }
}
 

Hello,

You can try to use the GetPosition method which works fine to me:

private void xamChart1_MouseMove(object sender, MouseEventArgs e)
{
            Point p = e.GetPosition(xamChart1);
            XValueTextBlock.Text = String.Format("X: {0:G5}", p.X);
            YValueTextBlock.Text = String.Format("Y: {0:G5}", p.Y);
}

I hope this helps

Vlad