I have done some testing on this and this is not possible using the reporting groupings, however I was able to come up with a solution using a custom form.
To accomplish this you would need to create a calculated custom form field with the calculation of: MONTH(Planned Completion Date). Be sure to set the Format of the field to 'number'. https://drop.workfront.com/v1uDem1l
This custom form would need to be attached to all object you want to report on. This calculation will capture the month of the Planned Completion Date. ie. 10/15/2020 would be 10 for October. You can then group by this custom field on your report and then chart off of these values.