There are two errors.
First, you're getting a diagonal line because the first input of line() contains all x-values but you're entering [x,y].
Second, there's no need to use the y-coodinate of the ginput().
Here's what you want:
line([x,x], ylim)
In addition to this, I suggest using the asix handle to ylim in case a different axis is current.
line([x,x], ylim(h)) %h is the handle to your gui axis.
