add dynamic vertical line to column chart

Step-by-step instruction in attachment. Basically: add dummy series and present it as scatterplot. It can be either 2-points: (4.5,0) and (4.5,18) or even just one point and error bar - I did it this way.

Final touch left for you in attachment -> change y for the point to 0 (I started from 1 because otherwise the red bar at first step would not be visible).

So the steps are:
add dummy series
changed dummy series to scatterplot type
edit dummy series, add X value (bars have only Y Value)
add error bars, subtype: custom
delete legend entry
do formatting of:
error bar
series point
vertical axis

Done!