protected override void OnRender(DrawingContext drawingContext)
{
  ...
  StreamGeometry geometry = new StreamGeometry();
  using (StreamGeometryContext context = geometry.Open())
  {
    context.BeginFigure(new Point(x, y), false, false);
    context.LineTo(new Point(x, y), true, true);
    ...
  }
  drawingContext.DrawGeometry(null, pen, geometry);
}