Tableau, strange behavior for reference line

Question

I'm attempting to show the results of an experiment. I'm using a calculated field and a parameter to create the following view, splitting the view by pre and post dates. The trouble is the "Average" reference line...

None of the reference lines are placed at the actual Average for that time period (or any time period, from what I can tell). I know this because when I highlight the full date range the reference line adjusts:

Here's how I've set up the Average:

here's the calculation for post-reset (pre-reset is the same). [ reset-date ] is a hard coded parameter (the date the intervention began).

date( if [Friday Date] >= [reset-date]
then [Friday Date]
end)

Any idea what's going on here?

Answer 1

I was helped on the Tableau forum itself: https://community.tableau.com/message/673000#673000

Instead of creating two date axis, I added one dimension to show pre and post.

See post at the link above for a full description, images, and a reference file.