totals trendline for stacked bar chart in Qlik Sense

[DutchArjo]

I have created a stacked bar chart in Qlik Sense and want a trendline for the totals.

I have searched for it and found some older topics about this subject (https://community.qlik.com/t5/QlikView-App-Dev/One-linear-trendline-on-stacked-bar-chart/m-p/710285) which as far as I know are not available in my current Qlik situation (Qlik Sense August 2023 patch 9).

How can I create a stacked bar chart and a trendline for the totals?

I started with a stacked bar chart and based on the older topics, changed to a combo-chart. I created the series for the stacked bar chart and a line chart for the totals. But I have no option to change this totals line chart. Should this option be available or is there any way I can achieve this?

combo chart with totals lineproperties for totals line