The horizontal left shows text on right side, and horizontal right shows text on the left side. They seem backwards.
The vertical top and bottom shows center.
DiagramNodeAnnotation annotation = new DiagramNodeAnnotation()
{
Content = shapeText,
Style = new AnnotationStyle()
{
Color = fontColor,
Fill = "transparent",
TextAlign = TextAlign.Center,
Bold = true
},
VerticalAlignment = VerticalAlignment.Bottom,
HorizontalAlignment = HorizontalAlignment.Left
};