Is there anything you wish to achieve using reference line on discrete date axis.
1. Convert your date to continuous part and using format option you can show relevant date.
You can see a black reference line which you can set to any desired value once you have date as continuous and using format option I have converted to quarter numbers 1,2,3,4 as Q1,Q2,Q3,Q4.
2. If you dont wish to convert X axis to continuous ; other option I can think of create calculated field and decide where you want to place reference in above example, I have created field to show Sales in August as reference line.
Create a dual axis chart. Change the mark size of your reference bar to make it appear as line.
Personally I would suggest 1st option in terms of convenience.
Please find the attached workbook for your reference.
Reference on Discrete Axis.twbx 461.0 KB