Hi Ajay Bajay, try this method.
1. In script, add a column for the reference line. In my case, I put the reference line at Feb 2008.
The additional column script:
Date(Monthstart(date('2/1/2008 12:00:00 AM')), 'MMM-YYYY') AS TargetMonth;
2. In the Presentation Tab of the chart, set the following expression for the Reference Lines:
3. Chart with reference line.