Hi Gus - you are correct - the reference line add-on fx expression - can be used to create a calculated, dynamic value to create a dynamic line as per the conditions defined in the expression. If you want to use a line as an actual trend line, you must use that formula you showed, as an expression in another measure.
Let me know if you have any other questions.
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I can't use your suggestions of the reference line as a measure since i have a simple bar chart and not a combo chart. And the client want to keep his sorting by weeks and year.
The goal was to add a line which would show each week's goals that the client could change. Kinda like so :
It's not possible with standard QlikSense (thanks Mikael) so does any of you know an extension that would do the trick?