def create_label(label_tuple, extra_label=None):
    """Return a label based on my_tuple (a,b) and extra label.

    a and b are string.

    The output will be something like:
                [a - b] extra_label
    """
    if extra_label is not None:
        return '[' + ' - '.join(label_tuple) + '] ' + str(extra_label)
    else:
        return '[' + ' - '.join(label_tuple) + ']'
Return a label based on my_tuple (a,b) and extra label.

    a and b are string.

    The output will be something like:
                [a - b] extra_label