It's hard to not think of chess here. Probability of each piece surviving to the last move of the game, averaged some database of chess games?

b*

And color relates to the probability of that piece issuing the checkmate?

The color is just a visual aid showing the probability -- lower probability, darker color. (And your guess would suggest that rooks issue checkmates less often than knights, which sounds wrong.)

But the king and queen pawns move in the first five turns with extremely high (>90%) probability.