Hi there Sebastian,
What you need to do is create a calculated field for the reference line that will exclude the Sales from the final year rather than just SUM(Sales).
In my example the years just go up to 2014 but you can just substitute in 2017. Just create a calculated field that will exclude any sales from the final year:
Add this calculated field to the marks card of your visualisation.
Then on your reference line change the value of the computation from SUM(Sales) to SUM(Name of your calculated field)
I have included the packaged workbook here as well. Hope that helps! Any more questions just let me know.
RefLine.twbx 1.2 MB
Thanks a lot, Perfect!