title: Python ValueError: x and y must have same first dimension
body: <p>I get the message ValueError: x and y must have same first dimension. Here is the code. Where is the mistake?</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib as mpl
  from scipy import signal
  import numpy as np
  
  x = np.linspace(0, 0.5, 200);
  y = 4* signal.sawtooth(2 * np.pi * 4 * k, 0.5)* signal.sawtooth(2 * np.pi * 4 * k, 0.5);
  
  plt.stem(x, y, use_line_collection=True);
  plt.show();
  </code></pre>