A third suggestion:
Dimension side selection: any of: top, left, bottom, or right.
The user might want only the horizontal dimension marked, as with a timeline, but might want the marks either above or below the object.
A third suggestion:
Dimension side selection: any of: top, left, bottom, or right.
The user might want only the horizontal dimension marked, as with a timeline, but might want the marks either above or below the object.