def METHOD_NAME(self, y_unit):
    if y_unit is None:
        return
    if self.spectral_unit.selected == "":
        # no spectral unit set yet, cannot determine equivalencies
        # setting the spectral unit will check len(flux_unit.choices) and call this manually
        # in the case that that is triggered second.
        return
    self.spectrum_viewer.set_plot_axes()
    if y_unit != self.flux_unit.selected:
        x_u = u.Unit(self.spectral_unit.selected)
        y_unit = _valid_glue_display_unit(y_unit, self.spectrum_viewer, 'y')
        y_u = u.Unit(y_unit)
        choices = create_flux_equivalencies_list(y_u, x_u)
        # ensure that original entry is in the list of choices
        if not np.any([y_u == u.Unit(choice) for choice in choices]):
            choices = [y_unit] + choices
        self.flux_unit.choices = choices
        self.flux_unit.selected = y_unit