Add 2 panels to the figure, top for signal and bottom for gene models

def panels(self):
        """
        Add 2 panels to the figure, top for signal and bottom for gene models
        """
        ax1 = self.fig.add_subplot(211)
        ax2 = self.fig.add_subplot(212, sharex=ax1)
        return (ax2, self.gene_panel), (ax1, self.signal_panel)