Can you send a screenshot of the worksheet used in this dashboard. Please include the entire Tableau screen not just the chart area.
Do you need the half years to be their own columns? Or do you simply want a line to mark where the half a year is?
can you share .twbx file with sample data?
Not exactly as your expected result, but sort of workaround if you can't find anything else here.
Use super store data as example attached.
Below is the steps and comments
Hope this helps
Example (20)_v10.2.twbx 2.4 MB