How to animate n curves in a plot using matplotlib's animation?

I have n curves that I want to draw using matplotlib’s animation (each curve corresponds to a gpx file recorded with a fitness tracker or a smartphone). It works well when using only one track or two tracks. But as soon as I want to adapt it to using n curves, I am lost. Here is my code:

import matplotlib.animation as anim
import matplotlib.pyplot as plt
import numpy as np

tracks  = {}
xdata   = {}
ydata   = {}

# in my case n_tracks would rather correspond to a couple of 100
n_tracks    = 2
n_waypts    = 100

for ii in range(n_tracks):
    # generate fake data
    lat_pts = np.linspace(10+ii*1,20+ii*1,n_waypts)
    lon_pts = np.linspace(10+ii*1,20+ii*1,n_waypts)

    tracks[str(ii)] = np.array( [lat_pts, lon_pts] )

    xdata[str(ii)]  = []
    ydata[str(ii)]  = []

fig = plt.figure()
ax1 = fig.add_subplot( 1,1,1, aspect='equal', xlim=(0,30), ylim=(0,30) )

plt_tracks  = [ax1.plot([], [], marker=',', linewidth=1)[0] for _ in range(n_tracks)]
plt_lastPos = [ax1.plot([], [], marker='o', linestyle='none')[0] for _ in range(n_tracks)]

def animate(i):
    # x and y values to be plotted
    for jj in range(n_tracks):
        xdata[str(jj)].append( tracks[str(jj)][0,i] )
        ydata[str(jj)].append( tracks[str(jj)][1,i] )

    # update x and y data
    for jj in range(n_tracks):
        plt_tracks[jj].set_data(  xdata[str(jj)][:],  ydata[str(jj)][:] )
        plt_lastPos[jj].set_data( xdata[str(jj)][-1], ydata[str(jj)][-1] )

    return plt_tracks, plt_lastPos

anim    = anim.FuncAnimation( fig, animate, frames=n_waypts, interval=20, blit=True )
plt.show()

The dictionary tracks contains the tracks, where for each track we have an array with longitude and an array with latitude data. The dictionary xdata and ydata is used for plotting purposes.

I have two lists with plotting objects, plt_tracks and plt_lastPos, where the first is used for successively plotting the track and the latter to indicate the latest position.

The error message reads RuntimeError: The animation function must return a sequence of Artist objects. So, my mistake seems to be the return statement, but simply adding a , at the end does not help here. Any hint on what I am missing would be greatly appreciated.