Trendline in Clustered LineChart

Dear Cognos Users!

I guess i have a problem in report studio, that can easily be solved (if someone has a bit of experience that i lack)…
In a Clustered Line Chart i have Categories (months) and a different Products as individual members.
somehow i am not able to create just a single trendline for one special product. in the Trendlines Menu (Chart Annotations) there is only the chance to select either trendlines for each product or one trendline for the average of all product of the Primary Axis…I also tried different Conditional Styles of Lines to make not wanted trendlines 100 % transparent - but this affected ALL LineTypes…
I am sure there is an easy way to solve this “problem” - can anyone help?
Thanks in advance

make a value prompt for products and use the productparameter in the linechart