python tips: draw poisson pdf graph with matplotlib

this is a code for drawing a graph of poisson pdf with matplotlib.

#!/usr/bin/env python                                                           
#coding: utf-8                                                                  
                                                                                
import pylab as pl                                                              
import numpy as np                                                              
                                                                                
def get_factorial(n):                                                           
    if n == 0:                                                                  
        return 1                                                                
    else:                                                                       
        return n * get_factorial(n-1)                                           
                                                                                
def get_probability(p, n, r):                                                   
    mew = float(n) * p                                                          
    denom = (mew ** r) * np.exp(-1 * mew)                                       
    molec = get_factorial(r)                                                    
                                                                                
    return denom/float(molec)                                                   
                                                                                
def plot_poisson_distrib(p, n, lt, lab):                                        
    xl = [nr for nr in range(0, 20, 1)]                                         
    yl = [get_probability(p, n, nr) for nr in range(0, 20, 1)]                  
    pl.plot(xl, yl, lt, label=lab)                                              
                                                                                
def main():                                                                     
    p = 0.02                                                                    
                                                                                
    mew = 1.0                                                                   
    plot_poisson_distrib(p, (mew / p), 'b-', 'P(%f,%f)' %(mew,p))               
                                                                                
    mew = 2.5                                                                   
    plot_poisson_distrib(p, (mew / p), 'r-', 'P(%f,%f)' %(mew,p))               
                                                                                
    mew = 5.0                                                                   
    plot_poisson_distrib(p, (mew / p), 'g-', 'P(%f,%f)' %(mew,p))               
                                                                                
    pl.ylabel('p(x)')                                                           
    pl.xlabel('x')                                                              
    pl.legend()                                                                 
    pl.show()                                                                   
                                                                                
if __name__ == "__main__":                                                      
    main()

and output of above code is: