The movement from selling 9 bottles of Coke to 7 bottles of Coke, as shown in the
graph, is the change in sales due to the substitution effect. We know this because it
is determined by keeping the consumer on the same indifference curve, and
comparing purchases at the two different prices. The price increase also has an
income effect, since it effectively lowers the consumer’s overall purchasing power.
Since Coke is a normal good, this lowering of income results in lower sales. Adding
this to the substitution effect means that sales will be less than 7.