luked:
One option is to hit the "x" key to select the x coordinate in the tracker after holding down the Shift key to constrain. Another option is to turn on Guidelines, a horizontal guideline will appear to constrain the direction, then tab into the x coordinate in the tracker.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David