Put value at centre of bins for histogram

Question

I have the following code to plot a histogram. The values in time_new are the hours when something occurred.

    time_new=[9, 23, 19, 9, 1, 2, 19, 5, 4, 20, 23, 10, 20, 5, 21, 17, 4, 13, 8, 13, 6, 19, 9, 14, 9, 10, 23, 19, 23, 20, 19, 6, 5, 24, 20, 19, 15, 14, 19, 14, 15, 21]

    hour_list = time_new
    print hour_list
    numbers=[x for x in xrange(0,24)]
    labels=map(lambda x: str(x), numbers)
    plt.xticks(numbers, labels)
    plt.xlim(0,24)
    pdb.set_trace()
    plt.hist(hour_list,bins=24)
    plt.show()

This produces a histogram, but the bins are not aligned as I would like. I want the hour to be in the centre of the bin, not on the edge.

I referred to this question / answer, but it seems not to answer the question either.

I tried the following code for the histogram plot instead, but it didn't plot a bar for the value 23

plt.hist(hour_list, bins=np.arange(24)-0.5)

Can anyone help me to get 24 bins, with the hour at the centre of each?

Answer 1

To get 24 bins, you need 25 values in your sequence defining bin edges. There are always n+1 edges for n bins.

So, alter your line

plt.hist(hour_list,bins=np.arange(24)-0.5)

to

plt.hist(hour_list,bins=np.arange(25)-0.5)

Note - your test data should have both edge cases in it. If you are simply extracting hours by rounding, there should be some 0 values in the list.

---

Full example:

import matplotlib.pyplot as plt
import numpy as np

def plot_my_time_based_histogram():
    #Note - changed the 24 values for 0
    time_new=[9, 23, 19, 9, 1, 2, 19, 5, 4, 20, 23, 10, 20, 5, 21, 17, 4, 13, 8, 13, 6, 19, 9, 14, 9, 10, 23, 19, 23, 20, 19, 6, 5, 0, 20, 19, 15, 14, 19, 14, 15, 21]
    fig, ax = plt.subplots()
    hour_list = time_new
    print hour_list
    numbers=[x for x in xrange(0,24)]
    labels=map(lambda x: str(x), numbers)
    plt.xticks(numbers, labels)
    #Make limit slightly lower to accommodate width of 0:00 bar
    plt.xlim(-0.5,24)
    plt.hist(hour_list,bins=np.arange(25)-0.5)

    # Further to comments, OP wants arbitrary labels too.
    labels=[str(t)+':00' for t in range(24)]
    ax.set_xticklabels(labels)
    plt.show()

plot_my_time_based_histogram()

Result: