im2 not defined in frequency.py

[SRimbaud]

Working with python 3.6.1

Sometimes in the function plotRidges the var im2 is ot defined at line 374. When there is no ridges the for loop doesn't define im2 which rises bugs.

        for r in self.ridges: #sometimes self.ridges is empty.
            points = np.array([self.t[::self.spacing][r[1]], self.f[r[0]]]).T.reshape(-1, 1, 2)
            segments = np.concatenate([points[:-1], points[1:]], axis=1)

            lc = LineCollection(segments, cmap=plt.get_cmap(cmap_chi),
                                norm=plt.Normalize(-np.pi / 4, np.pi / 4))
            lc.set_array(chi[(r[0], r[1])])
            lc.set_linewidth(5)
            im2 = ax[2].add_collection(lc)

        #im2 = ax[2].imshow(chi, vmin=-np.pi/4, vmax=np.pi/4,  interpolation='none', origin='lower', aspect='auto', cmap='coolwarm', extent=[self.t.min(), self.t.max(), 0, self.f[N/2-1]])

        # adjust figure
        fig.subplots_adjust(left=0.05, top=0.8, right=0.99, wspace=0.05)

        cbarax0 = fig.add_axes([0.05, 0.83, 0.303, 0.03])
        cbar0 = fig.colorbar(im0, cax=cbarax0, orientation='horizontal', ticks=[0, np.max(self.S0)])
        cbar0.ax.set_xticklabels(['', ''])
        cbar0.ax.xaxis.set_ticks_position('top')

        cbarax1 = fig.add_axes([0.369, 0.83, 0.303, 0.03])
        cbar1 = fig.colorbar(im1, cax=cbarax1, orientation='horizontal', ticks=[-np.pi/2, 0, np.pi/2])
        cbar1.ax.set_xticklabels([r'$-\frac{\pi}{2}$', r'$0$', r'$\frac{\pi}{2}$'])
        cbar1.ax.xaxis.set_ticks_position('top')

        cbarax2 = fig.add_axes([0.686, 0.83, 0.303, 0.03])
        ###Consequently im2 is not defined here when self.ridges is empty.
        cbar2 = fig.colorbar(im2, cax=cbarax2, ticks=[-np.pi/4, 0, np.pi/4], orientation='horizontal') 
        ###
        cbar2.ax.set_xticklabels([r'$-\frac{\pi}{4}$', r'$0$', r'$\frac{\pi}{4}$'])