well I guess you can try and "trick" the process but there is a minor risk involved.
create a formula field to zero out all of the vlaues that are not from this month called 'current_month_values' (or whatever)
if {table.datefield} in monthtodate then {table.valuefield} else 0
place the chart in the report header
on change of select your top 5 field and set the TopN to top 5
in the show value use the 'current_month_values' formula field
The risk here is that if you do not have at least 5 different on change of fields with a total >0 then the zeroed out fields frm other months come into play and will appear in the chart
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