Guide lines

Drag from a ruler to create a horizontal or vertical guide line. (Drag it back into the ruler to delete it.) This is positioned in document space. View > Guides should go next to the existing View > Rulers menu to toggle their visibility. Snapping works with them like they're a line (just without endpoints, since guide lines are infinitely long). Like everything else that you can drag in Graphite, Esc and right-click cancel a drag, putting it back where it began (or deleting it if this was just brought into existence by dragging out of a ruler).