Plot the diagonal.

def diag(ax=None, linecolor='0.0', linestyle='--', **kwargs):
    """Plot the diagonal."""
    ax = get_ax(ax)
    xy_min = np.min((ax.get_xlim(), ax.get_ylim()))
    xy_max = np.max((ax.get_ylim(), ax.get_xlim()))
    return ax.plot([xy_min, xy_max], [xy_min, xy_max],
                   ls=linestyle,
                   c=linecolor,
                   **kwargs)