Draw r-hat for each plotter.

def plot_rhat(self, ax, xt_labelsize, titlesize, markersize):
        """Draw r-hat for each plotter."""
        for plotter in self.plotters.values():
            for y, r_hat, color in plotter.r_hat():
                if r_hat is not None:
                    ax.plot(r_hat, y, "o", color=color, markersize=markersize, markeredgecolor="k")
        ax.set_xlim(left=0.9, right=2.1)
        ax.set_xticks([1, 2])
        ax.tick_params(labelsize=xt_labelsize)
        ax.set_title("r_hat", fontsize=titlesize, wrap=True)
        return ax