def join_plus(xs, check_non_empty=False):
    """
    Concatenate strings with '+' to form legend label.

    Parameters
    ----------
    xs : list
      List of strings
    check_non_empty : bool
      If True, assert that len(xs) > 0

    Returns
    -------
    str
        Concatenation of xs (e.g., "CPU + GPU")

    Examples
    --------
    >>> join_plus(["CPU", "GPU"])
    "CPU + GPU"
    """
    if check_non_empty:
        assert len(xs) != 0
    return ' + '.join(sorted(xs))
    # PROBLEM: this messes up the DQN plot for some reason...
    # return " +\n".join(sorted(xs))