Questions

Thanks for your comments!

Line Segment can be achieved with the LineAnnotation type. Please see our WPF Chart Annotations Example for what annotation types are available within SciChart WPF. The LineAnnotation documentation is here.

For a line that extends into infinity, it is possible with a small customization. Try this:

/// <summary>
/// Defines a read-only or editable Line annotation, which draws a Ray to the edges of the chart viewport, depending on two X,Y coordinates
/// 
/// e.g. if X Y coordinates are placed inside the chart, then the line extends until it hits the edge or the chart viewport
/// </summary>
public class RayAnnotation : LineAnnotation
{
    private Line _line;
    private Line _ghostLine;

    /// <summary>
    /// Initializes a new instance of the <see cref="RayAnnotation" /> class.
    /// </summary>
    public RayAnnotation()
    {
        // Reuse LineAnnotation control template from SciChart
        DefaultStyleKey = typeof(Abt.Controls.SciChart.Visuals.Annotations.LineAnnotation);
    }

    /// <summary>
    /// When overridden in a derived class, is invoked whenever application code or internal processes call <see cref="M:System.Windows.FrameworkElement.ApplyTemplate" />.
    /// </summary>
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        AnnotationRoot = GetAndAssertTemplateChild<Grid>("PART_LineAnnotationRoot");
        _line = GetAndAssertTemplateChild<Line>("PART_Line");
        _ghostLine = GetAndAssertTemplateChild<Line>("PART_GhostLine");
    }

    /// <summary>
    /// Override in derived classes to handle specific placement of the annotation at the given <see cref="AnnotationCoordinates" />
    /// </summary>
    /// <param name="coordinates">The normalised <see cref="AnnotationCoordinates" /></param>
    protected override void PlaceAnnotation(AnnotationCoordinates coordinates)
    {
        // Ray defined as having two handle points but extending to right hand side of edge of viewport
        _line.X1 = coordinates.X1Coord;
        _line.Y1 = coordinates.Y1Coord;
        _ghostLine.X1 = coordinates.X1Coord;
        _ghostLine.Y1 = coordinates.Y1Coord;

        // Calculate gradient of line
        double m = (coordinates.Y1Coord - coordinates.Y2Coord)/(coordinates.X1Coord - coordinates.X2Coord);

        if (double.IsNaN(m) || double.IsInfinity(m))
        {
            // Possible divide by zero above, just draw a single point for now
            _ghostLine.X2 = coordinates.X1Coord;
            _ghostLine.Y2 = coordinates.Y1Coord;
            _line.X2 = coordinates.X1Coord;
            _line.Y2 = coordinates.Y1Coord;
            return;
        }

        bool isRight = coordinates.X2Coord > coordinates.X1Coord;

        // Calulate projected X,Y point that touches the right/top edge, or left/bottom edge of the viewport
        double projectedX = isRight ? base.ParentSurface.ModifierSurface.ActualWidth : 0;
        double projectedY = coordinates.Y1Coord + m * (projectedX - coordinates.X1Coord);

        if (projectedY < 0)
        {
            bool isUp = coordinates.Y1Coord > coordinates.Y2Coord;

            // If the projected Y coord was above the top-edge, transpose the projection 
            // so that X is calulated and Y is set to zero (top edge)
            projectedY = isUp ? 0 : base.ParentSurface.ModifierSurface.ActualHeight;
            projectedX = coordinates.X1Coord - coordinates.Y1Coord/m;
        }

        Console.WriteLine("Projected X,Y = {0:0.00},{1:0.00}", projectedX, projectedY);

        // Set right-most edge of the line based on the calculated projection point
        _ghostLine.X2 = projectedX;
        _ghostLine.Y2 = projectedY;
        _line.X2 = projectedX;
        _line.Y2 = projectedY;
    }
}

       bool isRight = coordinates.X2Coord > coordinates.X1Coord;

        double projectedX1 = !isRight ? base.ParentSurface.ModifierSurface.ActualWidth : 0;
        double projectedY1 = coordinates.Y1Coord - m * (projectedX1 - coordinates.X1Coord);

        // Calulate projected X,Y point that touches the right/top edge, or left/bottom edge of the viewport
        double projectedX2 = isRight ? base.ParentSurface.ModifierSurface.ActualWidth : 0;
        double projectedY2 = coordinates.Y1Coord + m * (projectedX2 - coordinates.X1Coord);

        bool isUp = coordinates.Y1Coord > coordinates.Y2Coord;

        if (projectedY2 < 0)
        {


            // If the projected Y coord was above the top-edge, transpose the projection 
            // so that X is calulated and Y is set to zero (top edge)
            projectedY2 = isUp ? 0 : base.ParentSurface.ModifierSurface.ActualHeight;
            projectedX2 = coordinates.X1Coord - coordinates.Y1Coord / m;
        }

        if (projectedY1 > 0 && m != 0)
        {
            projectedY1 = !isUp ? 0 : base.ParentSurface.ModifierSurface.ActualHeight;
            projectedX1 = coordinates.X1Coord - coordinates.Y1Coord / m;
        }

        //Console.WriteLine("Projected X,Y = {0:0.00},{1:0.00}", projectedX2, projectedY2);


        GhostLine.X1 = projectedX1;
        GhostLine.Y1 = projectedY1;
        Line.X1 = projectedX1;
        Line.Y1 = projectedY1;

        // Set right-most edge of the line based on the calculated projection point
        GhostLine.X2 = projectedX2;
        GhostLine.Y2 = projectedY2;
        Line.X2 = projectedX2;
        Line.Y2 = projectedY2;

I have changed a few things but something seems off. I just reversed some of the logic. I would like to make it a full line, off to infinity in both directions. I feel that it is very close, just not quite there yet!