OK, fine, very pretty chart. NOW, tell me why Exxon and Shell do not compete for my business? It's a simple question.
You know, I don't understand what you're asking -- this is no surprise -- and I have trepidation about finding out.
They do, they just don't run sales. Sales are often a marketing gimmick designed to convince people to buy things they don't need, and most of the time a company that offers lots of sales and coupons is probably not giving you the best price year-round. Instead, they compete directly with the price, so you don't have to have the coupon or be there on the right weekend to get the best price.