option to add vertical line on plot x days/months since start of FUW or OW

[alexadima]

sometimes OW and FUW are calculated depending on an event (the date can be the same for all the sample, vary between patients).
it would be useful to have the possibility to draw a vertical line x days from FUW or OW to indicate this event, to be able to see how the medication events are situated in time in relation to this event. this is particularly useful for sliding window CMA

[ddediu]

This actually seems to be a particular case of a more general problem, that might need coordination with Sam (polypharmacy)

[ddediu]

... might be related to #100