python - matplotlib animated line plot stays empty

Question

I am trying to follow the basic animation tutorial located here and adapting it to display an already computed dataset instead of evaluating a function every frame, but am getting stuck. My dataset involves XY coordinates over time, contained in the lists satxpos and satypos I am trying to create an animation such that it traces a line starting at the beginning of the dataset through the end, displaying say 1 new point every 0.1 seconds. Any help with where I'm going wrong?

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

Code here creates satxpos and satypos as lists

fig = plt.figure()
ax = plt.axes(xlim=(-1e7,1e7), ylim = (-1e7,1e7))
line, = ax.plot([], [], lw=2)

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

def animate(i):
    line.set_data(satxpos[i], satypos[i])
    return line,

anim = animation.FuncAnimation(fig, animate, init_func=init,
                               frames = len(satxpos), interval = 1, blit=True)

Edit: The code runs without errors, but generates a blank plot window with no points/lines displayed and nothing animates. The dataset is generated correctly and views fine in a static plot.

Answer 1

In order to "trace a line starting at the beginning of the dataset through the end" you would index your arrays to contain one more element per timestep:

line.set_data(satxpos[:i], satypos[:i])

(Note the :!)

Everything else in the code looks fine, such that with the above manipulation you should get and extending line plot. You might then want to set interval to something greater than 1, as that would mean 1 millesecond timesteps (which might be a bit too fast). I guess using interval = 40 might be a good start.