Hello Peter -
In this example the reference lines are based upon the parameter Median 2018 and and Median 2019 - these are defined parameters as well and they are static. Why not utilize the median_year fields you have defined? Each can be placed on the marks card and utilized as a reference line.
The second issue relates the data only appearing when in a dual axis view. I have not been able to accomplish this within this view but i did find some useful suggestions such as - Need all same values on Tooltip for both line charts dual axis
I hope this helps.