The graph on the right plots the probability mass in the lone absorbing state that represents the final square as the transition matrix is raised to larger and larger powers.
A generic transition matrix in probability has a stationary distribution, which is the eventual probability to be found at any point no matter what the starting point.
The single parameter of this distribution (termed the "concentration parameter") controls the relative density or sparseness of the resulting transition matrix.
The transition matrix function is a function from the branch lengths (in some units of time, possibly in substitutions), to a matrix of conditional probabilities.
