You can add horizontal and vertical guidelines to your diagrams by clicking in the ruler areas along the top and side of the diagram layout screen and dragging your mouse to an appropriate area of your diagram. You can then use these straight lines as references for any other lines in your diagram. In addition, as you drag your line (or other shape) to the guideline(s) and the top and bottom edges and the midpoint of the shape approach the guideline, the edges and midpoint will snap to the line.
You can add as many guidelines to your diagram as you like. To delete a guideline, click on it and press the delete key.
For more information specifically regarding diagrams, check out the Diagramming Tutorial project, available in the Getting Started section of the program start page.