Lower Band.

    Formula:
    lb = cb * (1 - env_percentage)

def lower_band(data, period, env_percentage):
    """
    Lower Band.

    Formula:
    lb = cb * (1 - env_percentage)
    """
    cb = center_band(data, period)
    lb = [val * (1 - float(env_percentage)) for val in cb]
    return lb