In a [GIMP-user-mailinglist thread] the need for restricting tool movement 
along an axis came up. 
The basic use case is precise perspective correction of architecture photographs.
While trying to find solutions some other flaws became clear:
- The tools use each different interaction patterns for precise adjustment: 
  -- Move tool: restrict with Strg/Cmd key,
  -- Shear tool: restrict with the Strg/Cmd key, but only when pressed before shearing,
  -- Move, Shear, Scale, Rotate, Cage transform, Crop tool, Path tool: use guides
  -- Using the arrow keys only works for the Move tool and it only works for moving the active layer.
  -- Using the arrow keys is inconsistent = has different meanings for each tool. 
- Trying to align along guides works unprecise: the magnetic effect applies to the mouse cursor position, not a predictable, error-proof reference in the handle (e.g. the center). 
- The user is not able to create a reference path and a second path based on that reference path. He/She is always forced to edit the reference path.

Some ideas how to solve this come up: 
- Restrict all tools' movement with a modifier key, like Strg/Cmd.
- Restrict all pointer movement (not only of the tools) with a guide or path.
- Make snapping of handles to rulers/paths more predictable and error-proof.
- Restrict tool movement like the constraints in the new Unified Transform Tool
  (available in GIMP master).

The solution should  - if possible - also work with images that are much larger
than the screen. A typical use case is for instance the ever-growing size
of sensors in cameras and the resulting image dimensions. Another use case
is working with high resolution scans.

[GIMP-user-mailinglist thread]:
https://mail.gnome.org/archives/gimp-user-list/2013-October/msg00163.html