Histogram in matplotlib not working as intended

Question

I have two arrays:

import numpy as np
import pylab as pl

x = np.array([-36., -34.95522388, -33.91044776, -32.86567164,
   -31.82089552, -30.7761194 , -29.73134328, -28.68656716,
   -27.64179104, -26.59701493, -25.55223881, -24.50746269,
   -23.46268657, -22.41791045, -21.37313433, -20.32835821,
   -19.28358209, -18.23880597, -17.19402985, -16.14925373,
   -15.10447761, -14.05970149, -13.01492537, -11.97014925,
   -10.92537313,  -9.88059701,  -8.8358209 ,  -7.79104478,
    -6.74626866,  -5.70149254,  -4.65671642,  -3.6119403 ,
    -2.56716418,  -1.52238806,  -0.47761194,   0.56716418,
     1.6119403 ,   2.65671642,   3.70149254,   4.74626866,
     5.79104478,   6.8358209 ,   7.88059701,   8.92537313,
     9.97014925,  11.01492537,  12.05970149,  13.10447761,
    14.14925373,  15.19402985,  16.23880597,  17.28358209,
    18.32835821,  19.37313433,  20.41791045,  21.46268657,
    22.50746269,  23.55223881,  24.59701493,  25.64179104,
    26.68656716,  27.73134328,  28.7761194 ,  29.82089552,
    30.86567164,  31.91044776,  32.95522388,  34.        ])

y = np.array([ 28,  25,  30,  20,  32,  20,  10,  20,   9,  18,  10,   7,   7,
    14,  10,  11,   4,   8,   7,  11,   3,   7,   3,   1,   4,   3,
     1,   5,   1,   4,   1,   1,   1,  55,   2,   6,   2,   2,   5,
     5,   5,  10,  10,  17,  26,  28,  30,  34, 103, 137,  84,  59,
    55,  69,  59,  70,  72,  75,  66,  90,  79,  74,  62,  80,  59,
    62,  36,  43])

Both x and y have the same size. Now I want to plot a Histogram, where x represents the x axis and y the y axis. I try the following code:

pl.hist(y,x)

The resulting image is this one:

In this plot the maximum value goes up to seven, which does not make sense, since on the y array there are values as high as 137. The x array seems to be working, but I cannot figure out what is wrong with my y array.

I was following this example here:

Plot two histograms at the same time with matplotlib

What is your goal? Currently you are making a histogram of your y values where the x values define your bins, right? — plonser
– plonser, Commented Nov 17, 2015 at 16:46
It is working exactly as intended matplotlib.org/api/axes_api.html#matplotlib.axes.Axes.hist ;) — tacaswell
– tacaswell, Commented Nov 17, 2015 at 16:46
A "histogram" calculates the number of input values (x) within the bins (y). This is not what you want. — DilithiumMatrix
– DilithiumMatrix, Commented Nov 17, 2015 at 16:47

jme · Accepted Answer · 2015-11-17 16:46:29Z

9

Your data is already "binned", so-to-speak. plt.hist takes unbinned data, bins it, and plots a histogram. You simply need plt.bar:

>>> plt.bar(x, y)

Which gives:

answered Nov 17, 2015 at 16:46

jme

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MarianD · Accepted Answer · 2022-04-30 15:56:24Z

8

You are using the wrong function. You should be using pl.bar() as in http://matplotlib.org/examples/api/barchart_demo.html

What hist() does is take counts of the data in your vector and then plots bars of those counts. For example, if you have x=[1 1 3 2 5 5 5 2], then hist(x) will give a bar graph with height 2 at position 1, height 2 at position 2, height 1 at postion 3, height 0 at position 4 and height 3 at position 5.

edited Apr 30, 2022 at 15:56

MarianD

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answered Nov 17, 2015 at 16:45

honi

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