Hard disagree on both points. If the time line doesn’t matter, why include it? It would be simpler to just plot the endpoints.
As for the conclusion, what if McDonalds kept prices the same from 2004-2014, but Popeyes doubled prices over the same time period? The final plots for 2024 would be in a different order. It would be a different conclusion. Unless nobody changed prices at all before 2014, you’ll have a different final result.
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.