A crosstab might work better.
Use 2 formulas for grouping the data
One formula as "Month":
MonthName(month({table.documentdate}))
Another as "MonthYear":
totext({table.documentdateMM/yyyy")
In the crosstab add the the Month as the first grouping in Rows and @MonthYear as the second grouping rows then a SUmmary as a count of the record on a unique record field.
You may have to play with it to get the sorting to work since the Month field will sort alpha...