There seems to often be lapses in generations. We had the Federer, Safin, Roddick, Nalbandian, Hewitt, Ferrero, Davydenko generation born 81-82 then we had the 86-87 pack of talent with Djokovic, Nadal, Murray, Tsonga, Gasquet, Monfils and so on..
In between there seems to be almost nothing, players born in between generations like Ancic and Verdasco. I expect the next "new generation" of players to be born 91-92 or something like that, it seems they come every 5 years out of their cavern.
The graphic seems to suggest quite a long-term trend so I am not so sure that it's an issue of "generations." I think the older players are stronger physically, MORE EMOTIONALLY MATURE! during matches, MORE DISCIPLINED in SHOT SELECTION and have MORE VERSATILITY and VARIETY in their games. The young guns these days seem to come out and have somewhat one-dimensional, ball-bashing or flashy games but not enough tactical awareness, physical endurance, strength or discipline in matches. The older players play the match and not just the ball basically. The younger ones need more time to learn how to play smarter and get stronger. Andy Murray went out and got himself stronger but he already had the tactical intelliegence. The Gulbises, Monfilses, Cilices of the tour still have a lot to learn about tactics, versatility and conditioning IMO.