def add_scale_bar(apx, s, sl, c='y'):
    """
    Adds an aplpy scale bar
    :param apx: aplpy fig
    :param s: astropy.units quantity,  size (with units) of the scale bar
    :param sl: string, label for scale bar
    :param c: string, colour of scalebar
    :return:
    """
    apx.add_scalebar(s)
    apx.scalebar.set_label(sl)
    apx.scalebar.set_color(c)
    return apx