Python Matplotlib real time data plot problem

I tried to write a class, that would receive some value, and then it would plot in. Graph would be continuous line. The problem, however, is that graph values are always for some reason always 1 entry behind.

from matplotlib import pyplot as plt

class RealTimeGraph():
    x = []
    y = []

    def __init__(self, y_range = (), x_range = ()):
        self.index = self.index_gen()

        if (y_range):
            plt.ylim(*y_range)
        if (x_range):
            plt.xlim(*x_range)

    def index_gen(self, start = 0):
        while True:
            yield start
            start += 1

    def add_value(self, value):
        self.x.append(next(self.index))
        self.y.append(value)

        plt.cla()
        plt.plot(self.x, self.y)
        plt.pause(0.005)


if __name__ == '__main__':
    import random as r
    import time

    graph = RealTimeGraph(y_range = (0, 5))

    for i in range(10):
        graph.add_value(r.randint(0, 5))
        time.sleep(0.5)

Sleep function is simulating program, in which I want to use this class. What's causing this issue?