input: Write a python function to plot a graph using matplotlib.
  assert plot_graph([1, 2, 3], [1, 4, 9]) is None
  assert plot_graph([1, 2, 3, 4, 5], [1, 4, 9, 16, 25]) is None
output: def plot_graph(x, y):
      import matplotlib.pyplot as plt
      plt.plot(x, y)
      plt.xlabel('x-axis')
      plt.ylabel('y-axis')
      plt.title('Simple Plot')
      plt.grid(True)
      plt.show()