def rt_label(label, it = True):
    """
    Rich text label: Format label with italic + bold HTML tags and
     replace '_' by HTML subscript tags

     Parameters
     ----------

    label : string
        Name for the label to be converted, containing '_' for subscripts

    Returns
    -------

    html_label : string
        HTML - formatted label

    Examples
    --------

        >>> rt_label("F_SB")
        <b><i>F<sub>SB</sub></i></b>

    """
    if "_" in label:
        label = label.replace('_', '<sub>')
        label += "</sub>"
    if it:
        html_label = "<b><i>"+label+"</i></b>"
    else:
        html_label = "<b>"+label+"</b>"
    return html_label