Comparability, again, is at the heart of this chart. (Thanks Annette for bringing it up to my attention.)
What was attempted here was an impossible magic act, hoping to cram several disparate data series onto one chart. It would appear that the relative age of Agassi and Federer is considered the primary control variable. Alas, comparability is lost when the two sections were aligned by calendar year, rather than by age. For me, it would make more sense to compare Federer at 24 to Agassi at 24; that would involve comparing 2005 to 1994, for example.
In the junkchart version (below), I use what are, at first glance, pie charts. Ha, you muse, ain't I eating humble pie, given my repeated health warnings about pie charts? I plead not.
Here, I pick the pies because of their circular shape, which has a neat analogy with the annual, four-step series of Grand Slams. The reader does not have to judge the size of the slices or pies, only the shading and the location of each quarter-pie. By leveraging the pie analogy, the presentation is more compact. Compactness is a key virtue when the primary purpose of a chart is comparison; one would like to place the items for comparison as close together as possible.
As explained above, I use Age rather than Year as the variable. Quickly one observes that Agassi skipped many Grand Slams in his early career while Federer has played almost a full slate. They both won their first Slam at Wimbledon, both at 22 years old. The chart signs off with an intriguing question of whether Federer's future path would mirror Agassi's.
The other half of the chart is rather easier to manage. One would use Age as the variable and put both lines onto the same chart for compactness.
Reference: "Agassi Defies His Age; Federer Keeps on Rolling", New York Times, Sept 11, 2005.