MultiIndexChart_10.py

def create_reference_line(x, name, meta=""):
    """Returns a dictionary that defines a 
    black line bound to the secondary yaxis.
    """

    ref_line = dict(
        x=[x, x],
        y=[0, 1],
        name=name,
        yaxis="y2",
        mode="lines",
        type="scatter",
        line=dict(
            color="black",
            width=0.5
        ),
        meta=meta
    )
    return ref_line