Custom Week of Year (not FY nor calendar)?

Hello. My group sales manager has a dash to track running sales totals, with year over year by calendar year, so Jan to Dec. What he really wants is to track it May-April. Since our FY runs Sep-Aug, there's no built in way I can see for this. And I'm at a loss to figure how to get at it in a custom range. What am I missing?

We're trying to get a line graph where x-axis is order date by week of "year", with week1 starting May (so about calendar week 18), and work with a year offset so we can get YOY.

Thanks for any suggestions.

Sharon