The Walrasian demand function $x(p,w)$ satisfies the weak axiom of revealed preference if the following property holds for any two price wealth situations $(p,w)$ and $(p',w')$: If $p•x(p',w')\le w$ and $x(p',w')\ne x(p,w)$, then $p'•x(p,w)>w'$. Where $x$ is the amount of commodity, $p$ is price and $w$ is wealth.
The book(MWG's Microeconomics Theory, Page 29-30) says that the graph below satisfies the weak axiom. I have difficulty in understanding the graph. To be specific, I don't know why there is a dot in each of the budget set and I can't understand why it satisfies the weak axiom.