grid on/off isn't consistent when plotting multiple columns vs single

df=mkdf(100,2,data_gen_f=lambda r,c: r+ rand(1)*5,r_idx_type='i')
df.plot()
# vs
df.C_l0_g0.plot()

closed by 5b3656c.
now obeys MPL default style by default, like series.plot() does.