Animating a circle in Matplotlib Python

Question

I'm trying to animate a circle in such way that the circles size changes.
CODE

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

fig = plt.figure()
ax = plt.axes(xlim=(-3,3), ylim=(-2,2))
plt.axis('off')
line1, = ax.plot([], [], lw=2)
line2, = ax.plot([], [], lw=2)

def init():
    line1.set_data([], [])
    line2.set_data([], [])
    return line1, line2,

def animate(i):
    r = 0.0008*(i**2)-0.08*i+2
    x = np.linspace(-2, 2, 100)
    y = np.sqrt(r**2-x**2)
    line1.set_data(x,y)
    line2.set_data(x,-y)
    return line1, line2,

anim = animation.FuncAnimation(fig, animate, init_func=init, frames=100, interval=40, blit=True)

plt.show()

When I run this, all I see is some flickering. How can I fix this? and when I see some kind of circle, the edges of the 2 functions do not connect.

EDIT
I've edited the code above. Now I get a circle that isn't close because of the use of 2 functions. Is there a way to fix this? Or is it because Matplotlib doesn't have enough time to fully plot the 2 functions?

Answer 1

I imagine that your circle just goes out of frame very quickly, note that the input to the function animate is the timestep in the animation, as an integer, so here it would go from 0 to 100.