To show relationships between different shapes, you can use connections. To add a connection, mouse-over the second button on the right hand panel. A connection can be added to the canvas in the same way as a shape. If you would like to add a connection to a specific shape, simply hover over that shape and click-drag on one of the light blue anchor points surrounding it, and it will automatically be added.
Drag the free end of the connection to another shape to connect it to that shape. Small blue dots will appear on a shape to indicate anchor points that can be connected.
To further customize your connection, click on the connection and look at the connection Modifier. In the connection, the icon at the "tail" can be edited by altering Source, and the icon at the "head" can be altered by altering Target. The connection can also be customized in terms of color, stroke width, opacity, etc.
Updated about 1 month ago