The graph just gives a rough overview of how they've performed vs their team-mates in qualifying. Obviously you need to know who their team-mates have been in order to get an idea about it. You also need to consider the awful luck that Webber has in qualifying as well as racing.
Thinking about it more though, I'd say that Vettel has been better than Alonso in qualifying and the graph backs that up a bit. Webber isn't a bad qualifier, he's just a little bit slower/unluckier than Vettel, otherwise he'd be ahead of Kimi at least.
From the previous link, Before 2013 (no idea how accurate these are) -
Fernando Alonso outqualified his team-mates in 74% of all races
Nico Rosberg outqualified his team-mates in 72% of all races
Sebastian Vettel outqualified his team-mates in 72% of all races
Lewis Hamilton outqualified his team-mates in 72% of all races
Kimi Räikkonen outqualified his team-mates in 60% of all races
Mark Webber outqualified his team-mates in 53% of all races
Jenson Button outqualified his team-mates in 43% of all races
Felipe Massa outqualified his team-mates in 38% of all races
With Rosberg going backwards fast, the big 3 will be clear of anyone else. There's no question for me that over a lap Hamilton is clear best in F1 though. He beat Alonso in 2007 on his debut season, slapped Button down to the low 40%'s and now he's doing the same to Rosberg. If anyone can't accept that he's the best over a lap based on those snippets of information then they'll never be convinced.