Adding lines and measurements to a diagram

Click on the ruler icon located in the toolbar. Choose from a line, a ruler or an arrow then take the cursor to the diagram click and drag to place a line. 

Edit the length of the line or label an arrow in the panel.

Tip: Use the Shift key while dragging to have a line snap to a cardinal direction. 

Measurement Guides are a helpful way to see the distance between tables. Measurement guides will appear when selecting a table and moving it towards another table.