What I usually do (in MS Excel, never tried it with Calc) is that I add another data set and for the Y value, I take the highest value of all other series (so that the upper limit of the Y axis is not increased). As I often get the lines linking to the previous and next point, I remove the line and use the error bar (the min being 100%) so that it fakes a line.
Thanks to add '[Solved]' at beginning of your first post
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) if your issue has been fixed.
Edit: here is my proposal together with r4zoli's one below, based on his attachment (the advantage of my method is that you can put several vertical lines. But his method is quicker if you need only one.
AOO 4.1.2 on Xubuntu 15.10 (Wily Werewolf) and PortableApps port 4.1.2 on Windows 7