def diag_line(ax, **linekwds):
    """ Draw a diagonal line x=y"""
    linekwds.setdefault('ls', ':')   #Dotted ... 
    linekwds.setdefault('color', 'k') #black
    linekwds.setdefault('linewidth', 1)

    # Bisecting line
    ax.plot(ax.get_xlim(), 
            ax.get_ylim(), 
            **linekwds)    
    return ax