title: Scipy: poisson fit not working on particular data set
body: <p>I wrote a script to fit poisson distribution which seems to work well with a data set generated using python's random.poisson but it does't work on the data set I need to fit.
  Script is:</p>
  
  <pre><code>In [6]: from scipy.misc import factorial
  
  In [7]: from scipy.optimize import curve_fit
  
  In [8]: import numpy as np
  
  In [9]: import matplotlib.pyplot as plt
  
  
  In [11]: def poisson(k, lambd):
     ....:     print(lambd)
     ....:     return(lambd**k/factorial(k))*np.exp(-lambd)
  </code></pre>
  
  <p>Data is :</p>
  
  <pre><code>In [12]: x = [ 0.0036533, 0.00972361, 0.01579392, 0.02186422, 0.02793453, 0.03400484, 0.04007515, 0.04614546, 0.05221577, 0.05828608, 0.06435639, 0.0704267, 0.07649701, 0.08256731, 0.08863762]
  
  In [13]: y =[ 0.64005518, 0.10825634, 0.05954099, 0.04330254, 0.03383011, 0.02165127, 0.02435768, 0.01623845, 0.01082563, 0.00676602, 0.00947243, 0.00947243, 0.00270641, 0.00405961, 0.00947243]
  
  In [18]: popt, pcov = curve_fit(poisson, x, y)
  </code></pre>
  
  <p>So the problem is that best value of lambd found by this scipt(=2.82) is much higher than it should be. 
  Can someone help please?
  Thanks in advance</p>