I haven't tested this, but you might be able to convert all of the dates to the first of the month, use that actual date as the on change field, and then format the date label on the chart to show in MMM-yy format. To convert to the first of the month, you'll do something like this:
Date(Year({POPORH1.DATE}), Month({POPORH1.DATE}), 1)
-Dell