def whichplot(which, axe):
    """
    select plot function according to chosen log scale (if any).
    """
    if which == '':
        return axe.plot
    elif which == 'x':
        return axe.semilogx
    elif which == 'y':
        return axe.semilogy
    elif which == 'xy':
        return axe.loglog
