Matplotlib transparent point over transparent line

Question

I want to plot scatter points over a continuous line. The colors I chose have some value of alpha<1. When I draw the point on top of the line, the result becomes more opaque (which is expected). Here's a picture:

And here's the relevant source code:

import matplotlib.pyplot as plt
plt.style.use("default")
color   = (0.4, 0.1, 0.9, 1.0)
color50 = (0.4, 0.1, 0.9, 0.5)

# A line
fig, ax = plt.subplots(1,1,figsize=(6,4))
ax.plot([1,2],[1,2], lw = 10, color =color50)

# Point A
ax.scatter(1.4,1.4,  s = 500, color =color)
ax.text(1.38,1.47,"A",)

# Point B
ax.scatter(1.5,1.5,  s = 500, color =color50,alpha = 1.0)
ax.text(1.48,1.57,"B",)

# Point C
ax.scatter(1.7,1.7,  s = 500, color =color50)
ax.text(1.68,1.77,"C",)

# Point D
ax.scatter(1.7,1.3,  s = 500, color =color50)
ax.text(1.68,1.37,"D")

The result I desire would look somewhat like point D on the line without the line being more opaque (as it is the case for C).

Answer 1

With the help of scleronomic's comment and this link I found the solution:

import matplotlib.pyplot as plt
plt.style.use("default")
color   = (0.4, 0.1, 0.9, 1.0)
color50 = (0.4, 0.1, 0.9, 0.5)

# A line
fig, ax = plt.subplots(1,1,figsize=(6,4))
ax.plot([1,2],[1,2], lw = 10, color =color50)

# Point A
ax.scatter(1.4,1.4,  s = 500, color =color)
ax.text(1.38,1.47,"A",)

# Point B
ax.scatter(1.5,1.5,  s = 500, color =color50,alpha = 1.0)
ax.text(1.48,1.57,"B",)

# Point C
ax.scatter(1.7,1.7,  s = 500, color =color50,zorder=10)
ax.text(1.68,1.77,"C",)

# Point D
ax.scatter(1.7,1.3,  s = 500, color =color50)
ax.text(1.68,1.37,"D")


# The solution based on
# https://stackoverflow.com/questions/25668828/how-to-create-colour-gradient-in-python
import matplotlib as mpl
import numpy as np

def colorFader(c1,c2,mix=0):
    c1=np.array(mpl.colors.to_rgb(c1))
    c2=np.array(mpl.colors.to_rgb(c2))
    return mpl.colors.to_hex((1-mix)*c1 + mix*c2)

interpcolor = colorFader(color,(1.0,1.0,1.0),0.5)
# Point E
ax.scatter(1.8,1.8,  s = 500, color =interpcolor)
ax.text(1.78,1.87,"E")

# Point F
ax.scatter(1.9,1.9,  s = 500, color =interpcolor,zorder=3) # zorder>2
ax.text(1.88,1.97,"F")

I had to interpolate the color and change the zorder such that the scatter point is in the foreground.