N = 256
rgb = plt.cm.Blues(np.arange(N))
r, g, b, a = rgb[:,0], rgb[:,1], rgb[:,2], rgb[:,3]

fig, ax = plt.subplots(figsize=(6,3))
ax.plot(range(N), r, color='r', linewidth=1.4)
ax.plot(range(N), g, color='g', linewidth=1.4)
ax.plot(range(N), b, color='b', linewidth=1.4)
ax.plot(range(N), a, color='gray', linestyle='--', linewidth=1.4)
ax.axis((0,N-1,0,1))

plt.show()