Ive seen a few asks recently for a reference line that is different depending on the x axis values. While there is a feature request written, I came up with a cool way to make this work today using a combination of Combo Charts and Rollups.
Here you can see I added 2 additional Y axis values with a hardcoded split date. You can also use Between() or a date picker control to change how we determine the location of this split!
Just make sure to hide null data in the paintbrush menu for line style!