It's easy to see which players exclusively kill penalties (vertical axis), like Chris Kelly, Gregory Campbell and Daniel Paille up front and Johnny Boychuk and Andrew Ference on defense. These players obviously possess defensive talents, and their scoring totals will likely be modest. Incidentally, the bubbles are sized according to their even-strength playing time.
Similarly, players like Jaromir Jagr, David Krejci, Tyler Seguin, Nathan Horton and Milan Lucic up front and Dougie Hamilton on defense, are used exclusively to work power plays (horizontal axis). That provides a good confirmation of their offensive skills, and their scoring totals should therefore be excellent. Furthermore, the absence of Seguin, Horton and Jagr on the Bruins power play will be something to keep an eye on.
Then there's the players who work on both special teams, like Patrice Bergeron, Rich Peverley and Brad Marchand up front, and Zdeno Chara and Dennis Seidenberg on defense. These tend to be more complete two-way players, and very much the heart and soul of a team. In contrast there are the players of far more limited value, that work on neither of the special teams, like Wade Redden, Adam McQuaid, Jordan Caron and Shawn Thornton.