One method is to create at table of dates, called Months, by importing a sheet from Excel of a long range of YYYY-MM's which represents each month in a text format like 2017-1 (or 2017-01 if you think that reads easier).
Set the Key field of that table to be the YYYY-MM field.
On the child tables of Opps and Leads, make a field of YYYY-MM.
List("-", ToText(Year([lead date])), Right("0" & Month([Lead date]),2))
Then make a relationship back to the Months Table and make a summary field of the # of leads that month. Then repeat for Opps.
