Also another related thing is with "Straight snap" enabled you can also pull up vertical construction lines in the middle of drawing to help you snap on to points elevated in z directly in the initial drawing. See here for some more info on construction lines: http://moi3d.com/2.0/docs/moi_command_reference11.htm#constructionlines

and here is a video of this in action, note here that I'm drawing a curve with some points elevated in z directly in the initial curve drawing:




So note there that when I created the construction line there was a "z" showing from the base point, that's the indicator that straight snap was engaged and was helping me to track along a z direction line - that's how you bring things up in the z direction while in the 3D view. It will also track along the x or y directions as well if you move your mouse close to those directions instead, you just move nearby one of the directions and when you are close enough it engages and helps to snap things exactly in that direction.

So you use that same "Straight snap" tool for doing constrained movement in the x or y axis directions as well.

- Michael