Workfront doesn't let you chart/group on multi-select elements.
My usual workaround is to create a calculated field called Calculated FieldName with the Calculation:
IF(CONTAINS(", ",FieldName),"Multiple",FieldName)
So if there is a comma (ie more then one value), call it multiple, else just the single value from the checkbox. This can then be used to group and chart.
Melinda Layten
mlayten@gmail.com
Independent Workfront Consultant