Here is a trick I just thought of, then tried and it works:
Set the Wall height to zero and its thickness to zero.
If now you place such a Wall in 3D, it is actually a 3D line.
It works.
However, since the base of a Wall is always horizontal you will be able to draw only horizontal lines this way.
There seems to be one setting requirement: in the 3D Window Settings dialog, make sure Contours are not set to OFF. Then these Wall-lines will be displayed in 3D and there will be no error messages either.
I tried it and it works even in AC12, maybe also in even earlier version, I have not tried those.
If you want to use this trick, it may be a good idea to save a Wall Favorite with these settings so you can quickly activate whenever it is needed.
I think the reason drawing "lines" in 3D is very useful because then the SPACE-click method can be used on them quickly.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
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2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac28