def label_date(ax, label, date, df):
    """Helper function to annotate a date

    ``date`` is assumed to be in the index of ``df``

    Parameters
    ----------
    ax : Axes
       The axes to draw to

    label : str
        The text of the label

    date : object in index of df
        The x coordinate

    df : DataFrame
        The data source

    """
    y = df.loc[date]["mean"]
    return ax.annotate(
        label,
        (date, y),
        ha="right",
        xytext=(-10, -30),
        textcoords="offset points",
        arrowprops={"arrowstyle": "->"},
    )