When I do this, it only shows a straight line across where the "average" chg amount is. I'm wanting a "sloped" line that corresponds with the general trend of all the dots.
Does that clarify?
It depends on what you want to "average" across. You can select "Per Cell" to average across a group (in this case Site ID), which will show a "sloped" line similar to the 2016 Visits line.
Here is a description on the options you can select on the Reference Line dialog box