morph[morph.timestep < 131].groupby([
    'timestep',
    'Cell type'
]).count()['id_center'].unstack().plot(
    ls='--', ax=ax[1]
)

Oder irgendöppis i die richtig:

morph_filtered = morph[morph.timestep < 131]
morph_grouped = morph_filtered.groupby(['timestep', 'Cell type'])
morph_foo = morph_grouped.count()['id_center'].unstack()
morph_foo.plot(ls='--', ax=ax[1])