title: Finding first order derivative using central difference method
body: <p>so I have a set of data that is listed according to the first set of code, this part shouldn't be changed. My issue is trying that second part with the plot to perform a first order derivative using the central difference method below. I'm pretty sure the issue is with my central_diff function but I'm not sure about the rest.</p>
  <pre><code>import numpy as np
  data = np.loadtxt('angles.csv', delimiter=',', skiprows=1)
  stage = data[:,0]
  hip = data[:,1]
  knee = data[:,2]
  ankle = data[:,3]
  
  import matplotlib.pyplot as plt
  def central_diff(stage,knee):
      num = knee[1:]-knee[:1]
      den = stage[1:]-stage[:1]
      
      return num/den
  diff = central_diff(stage,knee)
  plt.plot(stage,knee,label='Analitical Solution')
  plt.plot(stage[1:1],diff, label='Central approximation')
  plt.legend()
  plt.show()
  </code></pre>