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.