How to partial fill_between in matplotlib, as in different colors for different values

Question

I'm trying to color the space between the graph line and the x-axis. The color should be based on the value of the corresponding point on the line. Kinda like the first graph: (https://matplotlib.org/3.1.1/gallery/lines_bars_and_markers/fill_between_demo.html)

If it's higher than 100 it should be red, if it's lower than 100 it should be green. The graph I'm looking for should be red-green alternating.

This is what I have at the moment:

import matplotlib.pyplot as plt

lenght = 120

y = [107, 108, 105, 109, 107, 106, 107, 109, 106, 106, 94, 93, 94, 93, 93, 94, 95, 106, 108, 109, 107, 107, 106, 108, 105, 108, 107, 106, 107, 97, 93, 96, 94, 96, 95, 94, 104, 107, 106, 108, 107, 107, 106, 107, 105, 107, 108, 105, 107, 100, 93, 94, 93, 95, 104, 107, 107, 108, 108, 107, 107, 107, 107, 104, 94, 96, 95, 96, 94, 95, 94, 100, 107, 107, 105, 107, 107, 109, 107, 108, 107, 105, 108, 108, 106, 97, 94, 94, 94, 94, 95, 94, 94, 94, 96, 108, 108, 107, 106, 107, 107, 108, 107, 106, 95, 95, 95, 94, 94, 96, 105, 108, 107, 106, 106, 108, 107, 108, 106, 107]

x = [x for x in range(lenght)]

lvl = lenght * [100]

fig, ax = plt.subplots()

ax.plot(x, y, color="black")
ax.fill_between(x, 0, y, where=y>lvl, facecolor='red', interpolate=True)
ax.fill_between(x, 0, y, where=y<=lvl, facecolor='green', interpolate=True)

plt.show()

This results in the following graph:

The area's where the value is less than 100 should be green. But the space between the line and the x-axis is always the color based on the first value in the array (in this example, red). How can I fix this?

Answer 1

Use numpy, A > B. Else, if not wanting to use numpy it would need to be [a > b for a,b in zip(A,B)].

import numpy as np
import matplotlib.pyplot as plt

y = [107, 108, 105, 109, 107, 106, 107, 109, 106, 106, 94, 93, 94, 93, 93, 94, 95, 106, 108, 
     109, 107, 107, 106, 108, 105, 108, 107, 106, 107, 97, 93, 96, 94, 96, 95, 94, 104, 107, 
     106, 108, 107, 107, 106, 107, 105, 107, 108, 105, 107, 100, 93, 94, 93, 95, 104, 107, 107, 
     108, 108, 107, 107, 107, 107, 104, 94, 96, 95, 96, 94, 95, 94, 100, 107, 107, 105, 107, 107, 
     109, 107, 108, 107, 105, 108, 108, 106, 97, 94, 94, 94, 94, 95, 94, 94, 94, 96, 108, 108, 107, 
     106, 107, 107, 108, 107, 106, 95, 95, 95, 94, 94, 96, 105, 108, 107, 106, 106, 108, 107, 
     108, 106, 107]
y = np.array(y)
x = np.arange(len(y))

lvl = 100

fig, ax = plt.subplots()

ax.plot(x, y, color="black")
ax.fill_between(x, 0, y, where=y>lvl, facecolor='red', interpolate=True)
ax.fill_between(x, 0, y, where=y<=lvl, facecolor='green', interpolate=True)

plt.show()