I'm teaching a class and I need a high-quality chart showing average
stock commission costs over time, preferably from the 1970s or so
until today.  This should be a color chart if possible with the axises
labeled.

JP and researchers -- fixed commissions disappeared in the 1970s. A
quick check at the following didn't reveal much.  It appears to be a
"historical" issue and tracking per share or per block commissions
isn't given much study today:
www.nyse.com
www.sec.gov
www.sia.com

The Securities Industry Association has a fair amount on commissions,
but from the point of view of registered professionals incomes.

This is one case where a book on the securities industry that's a few
years old might be the best source for data.

I appreciate the information but I'm still looking for a chart of some
type.  Commissions have fallen a lot since the 70s (when fixed
commissions were abolished), especially in the last few years with the
advent of online brokerages.  I need to illustrate that in a graphical
format and I am hoping I can find a chart somewhere on the Web that
will do this for me.