How to fix a target line independent of filter selection?

There is a beta feature in development called Filter Groups that will bring you the functionality you're describing.

In the immediate term, to support having a 'group average' while allowing filtering down to an individual you have to get clever with how you structure your data.

You can

- add group average as a column per individuals salesperson (I don't recommend, but it is the simplest to understand and least flexible after implemented).

- APPEND a copy of the entire dataset and fill "salesperson" with "default" then in filter cards always include 'default' and the selected salesperson.  All metrics must be built with an appropriate CASE statement.

- to avoid having to remember to include 'group average' in filters, and only select one salesperson:

  create a duplicate of the data for EACH salesperson, then label each duplicated section as one salesperson group.  use cases where the salesperson group = salesperson to identify when it's the numerator versus part of the denominator.