How do I get a fill_between shape in Funcanimation?

Question

I would like to make a moving plot where the area under the curve gets colored while the curve is getting plotted.

I have googled a bit and found that I should somehow create a patch. However, I do not understand any of the examples they give, so let me ask it here with my specific example:

import numpy as np
from matplotlib import pyplot as plt
from matplotlib import animation
import pylab as p

data = np.loadtext('datafile.dat', delimiter=',')
A = data[:,1]
B = data[:,2]

fig = plt.figure(figsize=(25,5), dpi=80)
ax = plt.axes(xlim=(0, 3.428), ylim=(-1,1))
line, = ax.plot([], [], lw=5)

def init():
    line.set_data([], [])
    return line,

def animate(i):
    x = A[0:(i-1)*400]
    y = B[0:(i-1)*400]
    line.set_data(x,y)
    # Here is the problem. I would now like to add the following line
    # p.fill_between(x, 0, y, facecolor = 'C0', alpha = 0.2)
    return line,

anim = animation.FuncAnimation(fig,animate, init_func=init, frames = 857, interval=20, blit=True)

I hope someone can give me a solution for my problem or at least point me in the correct direction.

So my question would be: how can I add the commented part without any errors?

Answer 1

Assuming you want blit = True, you will need to return the patch produced by fill_between as well.

p = plt.fill_between(x, y, 0)
return line, p,

Complete working example:

import numpy as np
from matplotlib import pyplot as plt
from matplotlib import animation

X = np.linspace(0,3.428, num=250)
Y = np.sin(X*3)

fig = plt.figure(figsize=(13,5), dpi=80)
ax = plt.axes(xlim=(0, 3.428), ylim=(-1,1))
line, = ax.plot([], [], lw=5)

def init():
    line.set_data([], [])
    return line,

def animate(i):
    x = X[0:(i-1)]
    y = Y[0:(i-1)]
    line.set_data(x,y)
    p = plt.fill_between(x, y, 0, facecolor = 'C0', alpha = 0.2)
    return line, p,

anim = animation.FuncAnimation(fig,animate, init_func=init, 
                               frames = 250, interval=20, blit=True)

plt.show()