i have a column of permit_number and first_approval_date
i need to show the bar report of average count  of permit_number per month wise

in that what can i do, for example if i need to add a new formula column for the average count of permit_number by first_approval_date - i need a formula for this,,
or
if i need to create a report i need suggestion how should i choose a x-axis and y-axis