We have analyzed your query and you can get X and Y value of the chart as shown in the below code snippet.
var mousePoint = new Point
X = e.GetPosition(SampleChart).X - SampleChart.SeriesClipRect.Left,
Y = e.GetPosition(SampleChart).Y - SampleChart.SeriesClipRect.Top
We have modified your sample based on the requirements and the sample can downloaded from the following location.
"PointerMoved" is an event for WinRT (it same as MouseMove in WPF). We have ported the WinRT code for all platforms. So we have added winrt code for this documentation. We will modify the documentation in our upcoming release.
Please let us know if you have any queries.