The timeline gives a general reference point. You're right they could mostly just include 2014 and the final year as well. No big deal in my opinion since they don't try to emphasize the price points at each year (as only a vague idea is given based on the line).

In terms of your second point, what if from '99 to 2003, Mcdonald's had doubled their prices? the graph would be the same? We can keep going back until each business's complete history, but I feel this graph tells enough of a story: Both brands increased their product costs a certain amount from the dates indicated.