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[To] show how players are used and how they performed at even-strength by mapping the percentage of shifts they started in the offensive zone (horizontal axis) against the average quality of their competition, as measured in attempted shots (vertical axis), with sized and coloured bubbles denoting how well the team performed with them on the ice relative to everyone else.This handy attachment includes in-depth explanation of how these graphs are put together and how to read them correctly, as well as analysis of last seasons numbers on a team-by-team basis.
When looking at the chart above, you can see (as the quote says) a Horizontal Axis representing Offensive Zone Starts in percentages.
Offensive zone starts is the percentage of all non-neutral shifts started in the offensive zone. A common misconception is that it's the percentage of all shifts started in the offensive zone, but it ignores those in the neutral zone and is therefore perhaps poorly named (like most hockey statistics). Think of it more as a representation of whether a player is used primarily for his offensive talents, or defensive.The Vertical Axis shows the Quality of Competition numbers (look at the raw data here at Behind the Net):
Quality of Competition is the average plus/minus of one's opponents over 60 minutes, except that it is based on attempted-shots (Corsi) instead of goals. In this particular variation we are using Relative Corsi (explained below). Players who face top lines will have high QoC's while those with the easier task of facing mostly depth lines will have negative QoCs.
What is Relative Corsi (The Bubbles)?
Corsi, another poorly named statistic, is simply a player's plus/minus, except that it's measured in attempted shots instead of goals. In this case it's calculated over 60 minutes, and Relative Corsi is calculated relative to how the team did without him.Again, you find all this info and more here.