Hi,
 
I managed to figure this out. I need to pass an init function to FuncAnimation. Quoting from the MatPlotLib API documentation:
 
init_func is a function used to draw a clear frame. If not given, the results of drawing from the first item in the frames sequence will be used.
 
So just in case someone has the same problem, here's the correct code:
 
import matplotlib
import numpy as np
from matplotlib.lines import Line2D
import matplotlib.pyplot as plt
import matplotlib.animation as animation

class Scope:
    def __init__(self, ax, maxt=10, dt=1):
        self.ax = ax
        self.dt = dt
        self.maxt = maxt
        self.tdata = [0]
        self.ydata = [0]
        self.line = Line2D(self.tdata, self.ydata, marker='o')
        self.ax.add_line(self.line)
        self.ax.set_ylim(0, 200)
        self.ax.set_xlim(0, self.maxt)
        self.ax.set_xlabel("Time(s)")
        self.ax.set_ylabel("Jitter Buffer Size(ms)", color=self.line.get_color())
 
    def init(self):
        self.line.set_data([], [])
        return self.line,

    def update(self, y):
        lastt = self.tdata[-1]
        if lastt == self.tdata[0] + self.maxt: # reset the arrays
            self.tdata = [self.tdata[-1]]
            self.ydata = [self.ydata[-1]]
            self.ax.set_xlim(self.tdata[0], self.tdata[0] + self.maxt)
            self.ax.figure.canvas.draw()

        t = self.tdata[-1] + self.dt
        self.tdata.append(t)
        self.ydata.append(y)
        self.line.set_data(self.tdata, self.ydata)
        return self.line,

def emitter():
    while True:
        yield np.random.randint(0,200)

fig = plt.figure()
ax = fig.add_subplot(111)
scope = Scope(ax)

# pass a generator in "emitter" to produce data for the update func
ani = animation.FuncAnimation(fig, scope.update, emitter, interval=1000, blit=True, init_func=scope.init)

plt.show()
Best Regards,
Clare.