def get_plot_dict_p_s(ch_var):
    """Returns a dictionary of key parameters for plotting based on varied component."""
    d = {}
    # polyelectrolyte density varied
    if ch_var == 'p':
        d = {'ch_var':'p', 'ch_fix':'s', 'order':[0,1], 'name_var':'Polymer'}
    # salt density varied
    elif ch_var == 's':
        d = {'ch_var':'s', 'ch_fix':'p', 'order':[1,0], 'name_var':'Salt'}
    else:
        print('invalid ch_var character: choose s or p.')

    return d