t = np.arange(50)
s1 = np.sin(t) * amp + mean
fig, ax1 = plt.subplots()
ax1.plot(t, s1, 'b-')
ax1.set_xlabel('time')
mn, mx = ax1.set_ylim(mean - amp, mean + amp)
ax1.set_ylabel('km$^3$/year')
km3yearToSv = 3.16887646e-05
ax2 = ax1.twinx()
ax2.set_ylim(mn * km3yearToSv, mx * km3yearToSv)
ax2.set_ylabel('Sv')
Plot single data with two Y axes (two units) in matplotlib