The perpendicular and the vertical lines are of course always bound to their axes. To extend them to the whole diagram, one has to add an new y-axis over the whole height and assign the events curve to it. By unchecking the visibility of all belonging elements (appearance, scale and axis labelings), it won’t be displayed.
The tedious part is to add this new axis parallel to the old ones. One has to disable the vertical stacking and to position the old axes per hand.