Draw effective n for each plotter.

def plot_neff(self, ax, xt_labelsize, titlesize, markersize):
        """Draw effective n for each plotter."""
        for plotter in self.plotters.values():
            for y, ess, color in plotter.ess():
                if ess is not None:
                    ax.plot(
                        ess,
                        y,
                        "o",
                        color=color,
                        clip_on=False,
                        markersize=markersize,
                        markeredgecolor="k",
                    )
        ax.set_xlim(left=0)
        ax.set_title("ess", fontsize=titlesize, wrap=True)
        ax.tick_params(labelsize=xt_labelsize)
        return ax