нужно использовать GuidelineSet:
protected override void OnRender(DrawingContext c)
{
base.OnRender(c);
Pen pen = new Pen(Brushes.Black, 1);
double h = this.ActualHeight;
double d = pen.Thickness / 2;
foreach (double x in new double[] { 7, 14 })
{
GuidelineSet g = new GuidelineSet(new double[] { x + d },
new double[] { 0 + d, h + d });
c.PushGuidelineSet(g);
c.DrawLine(pen, new Point(x, 0), new Point(x, h));
c.Pop();
}
}