Overall I think it's a fascinating chart, but I do have a couple of problems with it. One, it lists element 117 as undiscovered, despite the fact that it was synthesized by a joint U.S.-Russian team in 2010. Still, element 117 hasn't been officially accepted by the IUPAC/IUPAP, so I suppose the creators are just playing it safe.
But more importantly, I feel the chart's creators err when they assign the discovery of oxygen to England and Sweden. For the historical record, Carl Wilhelm Scheele of Sweden in 1772 and Joseph Priestley of England in 1774 both independently isolated oxygen by heating HgO and the like. Although Scheele performed his experiment first, Priestley published first, so there's some question as to which one deserves more credit-this chart seems to split the difference.
So far, so good. But there's a major issue. Both Scheele and Priestley subscribed to the phlogiston theory of combustion. Briefly, late alchemists/early chemists believed that a burning material gives off a substance known as phlogiston, which is absorbed by the air. The air will absorb phlogiston until it is saturated, at which point the flame will be extinguished. When it was observed that the substance created by heating HgO better supported combustion than regular air, Priestley and others concluded that it was "dephlogisticated air," which could absorb even more phlogiston that normal.
In 1777, the great chemist Antoine Laurent Lavoisier isolated oxygen in France. The extent of his knowledge of Scheele's and Priestley's prior work is contentious. In any event, the important point is that Lavoisier was an opponent of phlogiston theory, and correctly identified oxygen as a component element of air. Indeed, the term oxygen itself was coined by Laviosier based on his (incorrect) belief that all acids were based on oxygen.
So where does this leave us? Scheele and Priestley developed a technique, but their interpretations of its results were so flawed as to have no semblance of discovering oxygen. It was Lavoisier's theoretical work in identifying oxygen that can truly said to be its discovery. This is analogous to Newton's discovery of gravitation, though he (presumably) wasn't the first person to notice that things fall.
The discovery of oxygen was one of Thomas Kuhn's primary examples of the fact that discovery is not merely observing a fact, it's putting it in (to use Kuhn's terminology) its appropriate paradigm (for a nice review of Kuhn see here). Of course Lavoisier didn't get everything right about oxygen, but he placed it in the theoretical paradigm accepted by modern chemistry. In science, theoretical interpretation of experimental results is at least as important as the experiment in the first place. Without a paradigm, science is nothing more than a jumbled collection of observations.