Python Matplotlib pyplot histogram

Question

I am plotting a histogram of a fairly simple simulation. On the histogram the last two columns are merged and it looks odd. Please finde attached below the code and the plot result.

Thanks in advance!

   import numpy as np
   import random
   import matplotlib.pyplot as plt

   die = [1, 2, 3, 4, 5, 6]
   N = 100000
   results = []
   # first round
   for i in range(N):
       X1 = random.choice(die)
       if X1 > 4:
           results.append(X1)
       else:
           X2 = random.choice(die)
           if X2 > 3:
               results.append(X2)
           else:
               X3 = random.choice(die)
               results.append(X3)

   plt.hist(results)
   plt.ylabel('Count')
   plt.xlabel('Result');
   plt.title("Mean results: " + str(np.mean(results)))
   plt.show()

The output looks like this. I dont understand why the last two columns are stuck together.

Any help appreciated!

Answer 1

By default, matplotlib divides the range of the input into 10 equally sized bins. All bins span a half-open interval [x1,x2), but the rightmost bin includes the end of the range. Your range is [1,6], so your bins are [1,1.5), [1.5,2), ..., [5.5,6], so all the integers end up in the first, third, etc. odd-numbered bins, but the sixes end up in the tenth (even) bin.

To fix the layout, specify the bins:

# This will give you a slightly different layout with only 6 bars
plt.hist(results, bins=die + [7])
# This will simulate your original plot more closely, with empty bins in between
plt.hist(results, bins=np.arange(2, 14)/2)

The last bit generates the number sequence 2,3,...,13 and then divides each number by 2, which gives 1, 1.5, ..., 6.5 so that the last bin spans [6,6.5].