Yearly comics sales data comes in two varieties:
1) Year-by-year rankings of the sales figures found in comics publishers' Statements of Ownership, beginning in 1960. That material can be found here:
More years coming soon!
Some guesstimates at what the annual aggregate sales for some of these early years might be can be found here.
2) Aggregated monthly data, combining the preorders and final orders from Diamond Comic Distributors for each month from 1996 to present.
The below charts collect the Diamond data I've calculated across time. The widest grouping, visible below in black, has the widest margin of error; in addition to including the more solid estimates of comics-shop sales, guesses at returnable newsstand comics sales and returnable bookstore trade paperback sales are included.
The more reliable figures are the smaller groupings, appearing in red. These are calculated based on all the public reports issued by Diamond for comics shop sales in each month. The biggest such figure captures all comics, trade paperbacks, and magazines sold by Diamond in a year. As what Diamond has reported from year to year has changed, figures are not available for every category for every year.