How do I generate a histogram from a list of values using matplotlib?

Question

so I've been trying to plot a histogram using python with mathplotlib. So I've got two datasets, basically the heights of a sample of men and women as a list in python, imported off a csv file.

The code that I'm using:

import csv
import numpy as np
from matplotlib import pyplot as plt
men=[]
women=[]

with open('women.csv','r') as f:
    r1=csv.reader(f, delimiter=',')
    for row in r1:
        women+=[row[0]]

with open('men.csv','r') as f:
    r2=csv.reader(f, delimiter=',')
    for row in r2:
        men+=[row[0]]


fig = plt.figure()
ax = fig.add_subplot(111)

numBins = 20
ax.hist(men,numBins,color='blue',alpha=0.8)
ax.hist(women,numBins,color='red',alpha=0.8)
plt.show()

and the error that I get:

Traceback (most recent call last):
  File "//MEME/Users/Meme/Miniconda3/Lib/idlelib/test.py", line 22, in <module>
    ax.hist(men,numBins,color='blue',alpha=0.8)
  File "\\MEME\Users\Meme\Miniconda3\lib\site-packages\matplotlib\__init__.py", line 1811, in inner
    return func(ax, *args, **kwargs)
  File "\\MEME\Users\Meme\Miniconda3\lib\site-packages\matplotlib\axes\_axes.py", line 5983, in hist
    raise ValueError("color kwarg must have one color per dataset")
ValueError: color kwarg must have one color per dataset

I get the same error using that code. I believe the issue has something to do with how I imported the dataset? — Stefan de Souza
– Stefan de Souza, Commented Aug 10, 2016 at 12:35

TuanDT · Accepted Answer · 2016-08-10 12:36:43Z

1

NOTE:assume your files contain multiple lines (comma separated) and the first entry in each line is the height.

The bug is when you append "data" into the women and men list. row[0] is actually a string. Hence matplotlib is confused. I suggest you run this code before plotting (python 2):

import csv
import numpy as np
from matplotlib import pyplot as plt
men=[]
women=[]
import pdb;
with open('women.csv','r') as f:
    r1=csv.reader(f, delimiter=',')
    for row in r1:
        women+=[(row[0])]

with open('men.csv','r') as f:
    r2=csv.reader(f, delimiter=',')
    for row in r2:
        men+=[(row[0])]


fig = plt.figure()
ax = fig.add_subplot(111)
print men
print women
#numBins = 20
#ax.hist(men,numBins,color='blue',alpha=0.8)
#ax.hist(women,numBins,color='red',alpha=0.8)
#plt.show()

A sample output will be

['1','3','3']
['2','3','1']

So in the loops, you just do a conversion from string into float or integers e.g. women += [float(row[0])] and men += [float(row[0])]

answered Aug 10, 2016 at 12:36

TuanDT

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2 Comments

Stefan de Souza Over a year ago

Ah yes, it works perfect now! Thanks for the quick help!

TuanDT Over a year ago

Good to know it help! Could you please accept my answer by ticking the tick next to it? Thanks heaps. Happy coding

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