Use case:

I have a bar graph of sales $ by order date. I'd like to add a line that shows the average sales for the period shown. The "threshold" function creates the look that I want, but it seems to only accept a hard-coded value vs. a formula. I do not want to use a moving average, I want a horizontal line that runs the full length of the graph. It needs to update automatically, in case the user chooses to see last 7 days vs. last 30 days vs. last 90 days.

Is this possible?