Matplotlib: Animate function output in steps - animation

Question

I have a function that outputs return (x, y) and i would like to animate the x,y pair, from beginning to end. E.g. such that the line 'develops over time'.
This is what my output looks like:
x, y = stephan()
plt.plot(x,y)
And when I try to use a snippit of animation code:
fig = plt.figure()
ax = plt.axes(xlim=(0, 2), ylim=(-2, 2))
line, = ax.plot([], [])
def init():
line.set_data([], [])
return line,
def animate(i):
x, y = stephan()
line.set_data(x, y[i])
return line,
anim = animation.FuncAnimation(fig, animate, init_func=init,
frames=100, interval=20, blit=True)
I get this quite boring output:
It is plotting something, but certainly not the x,y output. I think that I might be using the animate or init function wrongly? And strangely enough, I haven't been able to find any code that does this quite simply.

Answer 1

If the call to stephan returns two floats, y[i] does not make any sense. Furthermore you would want to store the output in a list, to obtain a line and not a single dot per frame.
A working example could look like this
import matplotlib.pyplot as plt
import matplotlib.animation as animation
import numpy as np
class Stephan():
i=0
def __call__(self):
self.i += 0.02
return self.i, np.random.randn(1)
stephan = Stephan()
fig = plt.figure()
ax = plt.axes(xlim=(0, 2), ylim=(-2, 2))
line, = ax.plot([], [])
def init():
line.set_data([], [])
return line,
X = []
def animate(i):
x, y = stephan()
X.append((x,y))
line.set_data(zip(*X))
return line,
anim = animation.FuncAnimation(fig, animate, init_func=init,
frames=100, interval=20, blit=True)
plt.show()
Alternatively, if stephan returns two lists of values, you could directly set those lists as data to the line.
import matplotlib.pyplot as plt
import matplotlib.animation as animation
import numpy as np
class Stephan():
x=[0]
y=[1]
def __call__(self):
self.x.append(self.x[-1]+0.02)
self.y.append(np.random.randn(1))
return self.x, self.y
stephan = Stephan()
fig = plt.figure()
ax = plt.axes(xlim=(0, 2), ylim=(-2, 2))
line, = ax.plot([], [])
def init():
line.set_data([], [])
return line,
def animate(i):
x, y = stephan()
line.set_data(x,y)
return line,
anim = animation.FuncAnimation(fig, animate, init_func=init,
frames=100, interval=20, blit=True)
plt.show()