def _ylab(to_plot):
    """Returns the y-label for the plot given the type of plot.

    Parameters
    ----------
    to_plot : string
        Type of thing to plot. Can be 'pmf', 'cdf', 'fid', or 'wern'.

    Returns
    -------
    string
        The y-label for the plot.
    """
    labels_dict = {
        'pmf'  : "$\\Pr(T_n = t)$",
        'cdf'  : "$\\Pr(T_n \\leq t)$",
        'fid'  : "$F_n(t)$",
        'wern' : "$W_n(t)$"
    }
    return labels_dict[to_plot]