Customize the Ideal line for Release & Iteration Burndown Chart

I would like to customize the end date for the ideal line in the Release & iteration Burndown chart app.  The last iteration of a Release Increment is always a 'non-working' sprint where the teams work on hack-a-thon ideas and do the end of a release increment retrospective and beginning of a new release increment planning activities to fill in the 2-week timeframe.  With the ideal line including this Iteration, it can give a false sense of being on track as the ideal line goes out an extra 2 weeks of working time.  In the example below the release ends on 9/26/2018, but I'd like to have the ideal line end on 9/12/2018 instead to coincide with the actual working sprints of the release increment.