If you press and hold the [Shift] key while
placing a line, you can draw a line at an angle of 0 degrees and 90
degrees.
If you place a line while pressing the [Ctrl]
key, you can draw a line extending from the center. If you place a
line while pressing the [Ctrl] and [Shift] keys at the same time,
you can draw a line of 0 degrees or 90 degrees extending from the
center.
To edit after placing the line and polyline,
click on the selected line to change to a yellow handle. You can
drag the line to change the shape.
For a polyline, you can left click and drag to
draw, similar to handwriting.
or 
to place a
line/polyline on the screen. If you double-click the placed [Line]
or [Polyline], the following dialog box will appear.
,
,
,
or 
.