plot.py

#!/usr/bin/env python3
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm
import matplotlib as mpl

x = np.linspace(0, 10, 500)
y = np.cos(x)
value = 10 * np.cos(x / 3)
alpha = y**2

fig, ax = plt.subplots(figsize=(4, 3))

cmap = cm.get_cmap('hsv')
norm = mpl.colors.Normalize(vmin=-10, vmax=10)

C = np.zeros((len(x), 4))
C[:, :3] = cmap(norm(value))[:, :3]
C[:, 3] = alpha

ax.set(title='plot with correct colorbar and alpha')
ax.scatter(x, y, c=C)

sm = plt.cm.ScalarMappable(cmap=cmap, norm=norm)
sm.set_array([])
plt.colorbar(sm)

plt.show()