add a subplot annotation number

def annotate_subplot(ax, ncols=1, nrows=1, letter='a',
                     linear_offset=0.075, fontsize=8):
    '''add a subplot annotation number'''
    ax.text(-ncols*linear_offset, 1+nrows*linear_offset, letter,
        horizontalalignment='center',
        verticalalignment='center',
        fontsize=fontsize, fontweight='demibold',
        transform=ax.transAxes)