Project month numbering in Gantt chart

I have prepared a project Gantt chart for a funder who requires that that instead of actual calendar month (e.g. “September 2020”) the months are indicated by project month number: i.e. numbered 1-36 consecutively starting at project start (September 2020). Is this possible within OmniPlan as a custom format? Or would I be better exporting it to OmniGraffle and manually editing it?