How do I add a trenline to a selected potion of a line
 

How do I add a trenline to a selected potion of a line in a graph without
having to delete the other points in a graph

How do I add a trenline to a selected potion of a line
 

You don't.
Copy the series, and delete the points you don't want in the series.
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David Biddulph

T. Thamae wrote:
How do I add a trenline to a selected potion of a line in a graph
without having to delete the other points in a graph