Plot fractions of Eigenvalues sorted in descending order.

def plot_fracs(self, Q=None, ax=None, fignum=None):
        """
        Plot fractions of Eigenvalues sorted in descending order.
        """
        from ..plotting import Tango
        Tango.reset()
        col = Tango.nextMedium()
        if ax is None:
            fig = pylab.figure(fignum)
            ax = fig.add_subplot(111)
        if Q is None:
            Q = self.Q
        ticks = numpy.arange(Q)
        bar = ax.bar(ticks - .4, self.fracs[:Q], color=col)
        ax.set_xticks(ticks, map(lambda x: r"${}$".format(x), ticks + 1))
        ax.set_ylabel("Eigenvalue fraction")
        ax.set_xlabel("PC")
        ax.set_ylim(0, ax.get_ylim()[1])
        ax.set_xlim(ticks.min() - .5, ticks.max() + .5)
        try:
            pylab.tight_layout()
        except:
            pass
        return bar