How to center labels in histogram plot

Question

I have a numpy array results that looks like

[ 0.  2.  0.  0.  0.  0.  3.  0.  0.  0.  0.  0.  0.  0.  0.  2.  0.  0.
  0.  0.  0.  1.  0.  0.  0.  0.  0.  0.  0.  1.  0.  0.  0.  0.  0.  0.
  0.  1.  1.  0.  0.  0.  0.  2.  0.  3.  1.  0.  0.  2.  2.  0.  0.  0.
  0.  0.  0.  0.  0.  1.  1.  0.  0.  0.  0.  0.  0.  2.  0.  0.  0.  0.
  0.  1.  0.  0.  0.  0.  0.  0.  0.  0.  0.  3.  1.  0.  0.  0.  0.  0.
  0.  0.  0.  1.  0.  0.  0.  1.  2.  2.]

I would like to plot a histogram of it. I have tried

import matplotlib.pyplot as plt
plt.hist(results, bins=range(5))
plt.show()

This gives me a histogram with the x-axis labelled 0.0 0.5 1.0 1.5 2.0 2.5 3.0. 3.5 4.0.

I would like the x-axis to be labelled 0 1 2 3 instead with the labels in the center of each bar. How can you do that?

Answer 1

The following alternative solution is compatible with plt.hist() (and this has the advantage for instance that you can call it after a pandas.DataFrame.hist().

import numpy as np

def bins_labels(bins, **kwargs):
    bin_w = (max(bins) - min(bins)) / (len(bins) - 1)
    plt.xticks(np.arange(min(bins)+bin_w/2, max(bins), bin_w), bins, **kwargs)
    plt.xlim(bins[0], bins[-1])

(The last line is not strictly requested by the OP but it makes the output nicer)

This can be used as in:

import matplotlib.pyplot as plt
bins = range(5)
plt.hist(results, bins=bins)
bins_labels(bins, fontsize=20)
plt.show()

Answer 2

The other answers just don't do it for me. The benefit of using plt.bar over plt.hist is that bar can use align='center':

import numpy as np
import matplotlib.pyplot as plt

arr = np.array([ 0.,  2.,  0.,  0.,  0.,  0.,  3.,  0.,  0.,  0.,  0.,  0.,  0.,
        0.,  0.,  2.,  0.,  0.,  0.,  0.,  0.,  1.,  0.,  0.,  0.,  0.,
        0.,  0.,  0.,  1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  1.,  1.,
        0.,  0.,  0.,  0.,  2.,  0.,  3.,  1.,  0.,  0.,  2.,  2.,  0.,
        0.,  0.,  0.,  0.,  0.,  0.,  0.,  1.,  1.,  0.,  0.,  0.,  0.,
        0.,  0.,  2.,  0.,  0.,  0.,  0.,  0.,  1.,  0.,  0.,  0.,  0.,
        0.,  0.,  0.,  0.,  0.,  3.,  1.,  0.,  0.,  0.,  0.,  0.,  0.,
        0.,  0.,  1.,  0.,  0.,  0.,  1.,  2.,  2.])

labels, counts = np.unique(arr, return_counts=True)
plt.bar(labels, counts, align='center')
plt.gca().set_xticks(labels)
plt.show()

Answer 3

you can build a bar plot out of a np.histogram.

Consider this

his = np.histogram(a,bins=range(5))
fig, ax = plt.subplots()
offset = .4
plt.bar(his[1][1:],his[0])
ax.set_xticks(his[1][1:] + offset)
ax.set_xticklabels( ('1', '2', '3', '4') )

EDIT: in order to get the bars touching one another, one has to play with the width parameter.

 fig, ax = plt.subplots()
 offset = .5
 plt.bar(his[1][1:],his[0],width=1)
 ax.set_xticks(his[1][1:] + offset)
 ax.set_xticklabels( ('1', '2', '3', '4') )