A simple straight line can only have 2 magnet points – at head and tail.
A multi-segment line, however, has a magnet at each segment boundary.
One approach might be:
- View > Magnets
- Hold down the shift key to constrain line drawing angles to multiples of 45 degrees
- Make first click to start a straight line near the top of the canvas
- Move down the canvas (keeping the shift key down) clicking once for every additional straight line segment
- Dbl click to complete the line
You should now have a vertical line with magnets at various points.