The other day I stopped at a gas station here in rural Ohio.  I bought
a bag of chips and the guy charged me sales tax.  Was this legal?

I'm going to guess probably not.  Did you consume it on the premises?
Food consumed on the premises can be taxed (such as a restaurant).

http://tax.ohio.gov/divisions/communications/news_releases/news_release_062904.stm

"?Food? in Ohio is now defined as: substances, whether in liquid,
concentrated, solid, frozen, dried or dehydrated form, that are sold
for ingestion or chewing by humans and are consumed for their taste
and nutritional value."

"Any item with a ?Nutrition Facts? box on the label, including
nutritional and diet products (e.g. Ensure®, SlimFast®, etc.), is
generally considered to be food and it is not taxable. Any item with a
?Supplement Facts? box on the label, including items sold as dietary
supplements, is generally not considered to be food and so is subject
to sales tax."

Perhaps the gas station is making a judgment that the chips have no
taste or nutritional value?

Answering from the land of taxation, Canada....

I looked up Ohio Sales Tax http://tax.ohio.gov/divisions/sales_and_use/index.stm

There is a PDF explaining what 'foods' are taxable.  
http://pdftohtml.markoer.org/pdf2html.php?url=http://tax.ohio.gov/divisions/sales_and_use/documents/streamlined_sales_tax_food_BR.pdf

You definately should not have been taxed based on this document.

You can contact the Ohio Tax department here: 
 Questions? Call toll-free
1-800-304-3211*
or
Taxpayer Services
1-888-405-4039

BTW...if you bought the chips and consumed them on site they WOULD be
taxable!  But how would the retailer know before you paid where you
were going to eat them!