To make a line have more than one segment, select the line, then press the "New line point" tool. Create as many new control points (and therefore segments) as you like. Then arrange the points into a rough approximation of how they should be laid out horizontally and vertically. Click on "Straighten lines" to tidy up the layout.
To delete a line control point, select the line and click on "Cut line point" tool. An arbitrary point will be deleted.