See attached .QVF - copy to: C:\Users\<user profile>\Documents\Qlik\Sense\Apps, restart Sense or press F5 - in the hub after copying if Sense is open. Launch the Trend app.

Please let me know if this is what you are after.

The expression below needs to be used and calculated as a new measure in combination with a combo chart:

linest_m(total aggr(if(sum(Sales),sum(Sales)),OrderDate),OrderDate)*
only({1}OrderDate)+linest_b(total aggr(if(sum(Sales),sum(Sales)),OrderDate),OrderDate)