Hello Roelof,

There are a few ways to achieve your requirement.

1. I recommend handling the following events on the DataChart

a. SeriesMouseLeftButtonDown
b. PreviewMouseMove
c. PreviewMouseUp

You were on the right path. You may reposition the data points via the chart's GetUnscaledValue method to change the x and y positions.

eg.

  private void XChart_SeriesMouseLeftButtonDown(object sender, DataChartMouseButtonEventArgs e)
        {
            this.DraggingWidget = e.Item as Widget;
        }

private void XChart_PreviewMouseMove(object sender, MouseEventArgs e)
        {
            if (this.DraggingWidget != null)
            {
                Point p = e.GetPosition(xChart);

                this.DraggingWidget.ValueX = this.xChart.Axes.OfType<NumericXAxis>().First().GetUnscaledValue(p.X, this.xChart.WindowRect, this.xChart.ViewportRect);
                this.DraggingWidget.ValueY = this.xChart.Axes.OfType<NumericYAxis>().First().GetUnscaledValue(p.Y, this.xChart.WindowRect, this.xChart.ViewportRect);
            }
        }

 private void XChart_PreviewMouseUp(object sender, MouseButtonEventArgs e)
        {
            this.DraggingWidget = null;
        }

I've attached a complete sample demonstrating this.

2. You may also find the following forum post useful, since it demonstrates using a behavior.

https://ko.infragistics.com/community/forums/t/81022.aspx

Let me know if you have any questions.

For more details please visit msdn:
https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.100).aspx

eg.

 public class Widget : INotifyPropertyChanged
    {
        private double _ValueX;
        public double ValueX
        {
            get { return this._ValueX; }
            set
            {
                bool changed = this.ValueX != value;
                if (changed)
                {
                    this._ValueX = value;
                    NotifyPropertyChanged("ValueX");
                }
            }
        }
        private double _ValueY;
        public double ValueY
        {
            get { return this._ValueY; }
            set
            {
                bool changed = this.ValueY != value;
                if (changed)
                {
                    this._ValueY = value;
                    NotifyPropertyChanged("ValueY");
                }
            }
        }
        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged(String info)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(info));
            }
        }

You can suggest new product ideas for future versions by emailing ideas@infragistics.com.

Submitting your idea will send it directly to our product management team so that it can be imported into our new ideas community once live: http://ideas.infragistics.com

Remember when submitting your idea to explain the context in which a feature would be used and why it is needed as well as anything that would prevent you from accomplishing this today. You can even add screenshots to build a stronger case.  You can also link back to this thread for additional details.
Thank you in advance to submitting your product idea.