Buy vs Rent chart is not net present valued... not sure if that was your question though?
if it is not net present valued then surely the calculations are off - won't that push the ownership beats renting even further out in time?
Also I did not see if it was interest only vs repayment (one of the biggest issues IMO)
However, when we sum future expenses or cash flows (across years) and present them as a single number, generally in today's money, those values are all PV'd. For example, on our mortgage comparison page the total expenses over the amount of time you plan on holding the mortgage is a number we PV.