Allows you to create a single keypoint line.

If you click the endpoint of an existing keypoint curve, the new line will attach itself to the end of the existing curve. Dragging the common endpoint will move both curves.

to add the point freely.
to align the point horizontally
to the first point.
to align the point vertically
to the first point.
As you drag the end point, the Line tool displays the length of the line in current linear units.