Plot the equation of state on axis `ax`

        Args:
            ax: matplotlib :class:`Axes` or None if a new figure should be created.
            fontsize: Legend fontsize.
            color (str): plot color.
            label (str): Plot label
            text (str): Legend text (options)

        Returns:
            Matplotlib figure object.

def plot_ax(self, ax=None, fontsize=12, **kwargs):
        """
        Plot the equation of state on axis `ax`

        Args:
            ax: matplotlib :class:`Axes` or None if a new figure should be created.
            fontsize: Legend fontsize.
            color (str): plot color.
            label (str): Plot label
            text (str): Legend text (options)

        Returns:
            Matplotlib figure object.
        """
        ax, fig, plt = get_ax_fig_plt(ax=ax)

        color = kwargs.get("color", "r")
        label = kwargs.get("label", "{} fit".format(self.__class__.__name__))
        lines = ["Equation of State: %s" % self.__class__.__name__,
                 "Minimum energy = %1.2f eV" % self.e0,
                 "Minimum or reference volume = %1.2f Ang^3" % self.v0,
                 "Bulk modulus = %1.2f eV/Ang^3 = %1.2f GPa" %
                 (self.b0, self.b0_GPa),
                 "Derivative of bulk modulus wrt pressure = %1.2f" % self.b1]
        text = "\n".join(lines)
        text = kwargs.get("text", text)

        # Plot input data.
        ax.plot(self.volumes, self.energies, linestyle="None", marker="o", color=color)

        # Plot eos fit.
        vmin, vmax = min(self.volumes), max(self.volumes)
        vmin, vmax = (vmin - 0.01 * abs(vmin), vmax + 0.01 * abs(vmax))
        vfit = np.linspace(vmin, vmax, 100)

        ax.plot(vfit, self.func(vfit), linestyle="dashed", color=color, label=label)

        ax.grid(True)
        ax.set_xlabel("Volume $\\AA^3$")
        ax.set_ylabel("Energy (eV)")
        ax.legend(loc="best", shadow=True)
        # Add text with fit parameters.
        ax.text(0.5, 0.5, text, fontsize=fontsize, horizontalalignment='center',
            verticalalignment='center', transform=ax.transAxes)

        return fig