Thank you for responding, but no, that wont work. I need to get the axis coordinates of *any* mouse event on the chart, not just those concerning data items. I eventually worked out the following hack, which I hope will continue to work. It seems like this should be implemented properly by one of your methods but I just couldn't find it:

 private Point ConvertMouseToAxisCoordinates(Point p)
        {
            // MAJOR HAAXXX -- but gridarea dimensions seem to be more reliable than the axis ones
            double aw = chart.Scene.GridArea.ActualWidth;
            double ah = chart.Scene.GridArea.ActualHeight;
            p.X = p.X * ((xaxis.Maximum - xaxis.Minimum) / aw) + xaxis.Minimum;
            p.Y = -p.Y * ((yaxis.Maximum - yaxis.Minimum) / ah) + yaxis.Minimum;
            p.X = Math.Max(xaxis.Minimum, Math.Min(xaxis.Maximum, p.X));
            return p;
        }

  private void InsetChart_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            chart.CaptureMouse();
             Point p = e.GetPosition(xaxis);

             p = ConvertMouseToAxisCoordinates(p);
             .....do stuff with point like update a marker or add datapoints

}