def journey_data_years(all_y = False):
    """Return available journey data years
    
    Either return all the years, if all_y = True, or just
    those that are relevant from the TTM viewpoint.
    """
    
    if not (all_y):
        return (2012, 2014, 2015, 2016)
    else:
        return (2007, 2009, 2010, 2012, 2014, 2015, 2016)