Thanks a lot Jon,

That's it. I did not realize that "" is in fact a string. And I did my small
data set testing using "no data" instead of "" which I thought meant the same
thing. So I got confused by the results.

So, how should I actually do IF(Condition;Something;"") then?
The reason I use it is that it works well on the Y-data, and that the table
I plot from looks "as clean as it is" (I don't want to fill all empty cells
with "########").

Is there a workaround, or even some way to force the plot to "think right".

Thank's again for clearing this up / Ake