One way would be to make 12 charts, 6 for calendar year, 6 for fiscal year. You can have another date field of which year start would be set to April (you can do this by right clicking on date field -> Default Properties -> Fiscal Year starting -> choose month) and use this date field in your 6 fiscal year charts. Then dynamically switch these 2 types charts according to what is selected in parameter.
The idea is good, but I need to create the entire dashboard again. It will be a re-work for me. And also there are so many (approx 40) charts involved in the dashboard.
Please let me know if there is any other better way to implement this.