With the “Line” shape, you can create horizontal or vertical lines by default. The orientation depends on if the bounding box is larger in horizontal or vertical orientation.

Moreover, you can create user-specific lines by setting the amount of control points to 2 or higher in the properties.

The control points can be moved by keeping the mouse button pressed. You can add additional control points by double-clicking on a control point. By double-clicking and keeping the <Shift> key pressed, you can delete control points.