There are known issues with Windows OS when Display settings not set to 100%.
Does this line also appear when Display setup is 100%?
Indeed the line does not show. This was also the solution for another issue I had. The scroll bars weren't displayed in the relationship graph when the setting was at 125%..
sad to say but this stays as a unresolved FM bug that gives us a great headache
For More Information see: FM13v02 - with fatal rendering issues!!!
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip