title: Plotting exponential function python
body: <p>I get a linear graph when trying to plot exponential function:</p>
  
  <pre><code>import math
  import numpy as np
  import matplotlib.pyplot as plt
  
  def graph(formula, x_range):
     x = np.array(x_range)
     y = eval(formula)
     plt.plot(x, y)
  
  graph('100*(np.power(0.8, x))', (0,100))
  </code></pre>
  
  <p>what am I doing wrong?
  <a href="http://i.stack.imgur.com/HEecP.png" rel="nofollow">output_image</a></p>