Thank you, mattness!
Unfortunately this problem is still actual for me. I don't have any time criteria - and will not have... Anyway, I needed it to create a dashboard based on it. So, here are the way around which I have found: Just create a pivot using required data with any chart type (don't pay serious attention to it on this step) and save it as dashboard - when you open it on dashboard you will be able to change your chart type there as you would like to.. and without any blockers! =)))
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