As of 3.18, it is now possible to create a reference line through the UI on the Explore page. Once you have a completed query, open the Visualization tab and the Visualization Configuration tab from the gear button on the right. Select "Y" for Y-Axis options, then click "Add Reference Line":
NOTE: You are able to add as many reference lines as you would like to each visualization
You can then specify what type of reference you'd like:
What you'd like the value to be:
And how you'd like it to appear:
Note: The Label Position option is available starting in release 3.50.
That will lead to the final product: