I need a dynamic line that changes according of the number of items.
How I wanted:
Not sure, but you could try to place a SubReport below your last detail (e.g. dummy group footer or summary) and set its attribute "RunToBottom" to "true".
In this "Dummy SubReport" you just place a line into the background with attribute "stretchType" to "StretchRelativeToBandHeight".
So not for sure... but worth to give it a try! ;-)
Hth + regards