The figures are adjusted for prices and expressed in 2011 dollars. As such, any number (e.g. $12k) on the graph, whether it be in 1980, 90s, 000s, 2010, should represent the same buying power.
Calculations and methodologies can be off, of course. Purchasing power is part objective, part subjective. (e.g. how to compare a computer that is 10% more expensive, can do has 500% more calculations per second, but also just runs MS Office mostly the same compared to last year's computer... you can have different methodologies, one which says prices went up by 10%, others prices went down by almost 80%). But in principle, if you believe the figures are correct, then buying power has been taken into account.
Calculations and methodologies can be off, of course. Purchasing power is part objective, part subjective. (e.g. how to compare a computer that is 10% more expensive, can do has 500% more calculations per second, but also just runs MS Office mostly the same compared to last year's computer... you can have different methodologies, one which says prices went up by 10%, others prices went down by almost 80%). But in principle, if you believe the figures are correct, then buying power has been taken into account.