Define a summary field that counts a never empty field such as exam date in your table.
I will call the above calculation field, cMonth. Make sure that "date" is specified as the field's return type.
You can use this expression in the chart setup to label your x-axis:
Left(MonthName ( YourTable::exam Date ) ; 3 ) & Right ( Year ( YourTable::exam date ) ; 2 )
Use the summary field as the field for your Y-Axis
Make sure that your data source settings are for "found set", "groups when sorted".
Then be sure to sort your records by cMonth.