Plotting in python3 (histogram)

Question

I am trying to create a histogram from an array of grades. All the grades are one of the possible 7-scale (-3, 0, 2, 4, 7, 10, 12).

I am using the below code to generate the plot, however I cannot identify a way to put the x-axis label on the middle of the bars, remove the space in the middle of the plot and maybe put a small space between the bars...

import matplotlib.pyplot as plt
finalGrades = [-3, -3, 10, 2, 10, 0, 7, 7, 12, -3, 7, 0, 12, 12, 12 ,12, 12, 0, 0, 0, 4]
plt.hist(finalGrades, bins=8)
plt.xticks([-3, 0, 2, 4, 7, 10, 12])
plt.title("Final Grades plot")
plt.xlabel("All possible grades")
plt.ylabel("Number of students")
plt.show()

The plot is correct. It shows 8 equally distributed bins between -3 and 12. The x labels are set exactly as you coded it. So, you may need to explain better what you want to achieve. Do you want unequally distributed bins or just name the bins by a number which is arbitrarily chosen? — ImportanceOfBeingErnest, Nov 09 '16 at 16:11
@ImportanceOfBeingErnest I would the x-axis to show the numbers in the middle of the bar - as I have edited (*sorry, in paint!*) in red in the image below: [link](http://imgur.com/a/pSjS4) — Silverthorn, Nov 09 '16 at 19:26

score 4 · Accepted Answer · edited May 23 '17 at 12:34

One needs to understand that the numbers cannot be in the middle of the bar, since they are at exactly those positions where they should be. (If you put 0 as close to -3 as to 2, the gods of mathematics will start crying.)

So what you are asking for here is a histogram of categorial values, not numerical ones. There are some questions related to categorial histograms already, see, e.g.

What you need to do, is to think of the grades -3, 0 ,2 etc. as category (like red, green, yellow) and the question is now how often each category is represented in the list finalGrades. Since matplotlib.hist only understands numerical data, we would map the n categories to the first n integers, -3 -> 0, 0 -> 1, 2 -> 2 and so forth.
Now instead of a list of grades
[-3, -3, 10, 2, 10, 0, ...]
we have a list of category numbers
[0, 0, 5, 2, 5, 1, ...]
and those category numbers are equally spaced, such that the histogram will understand what we want. The histogram can then be plotted with bins = [0,1,2, ... , 6,7] (we need 8 bin edges to get 7 bins). Finally, and funnily enough, align='left' makes the bins centered at the tickmarks. The tickmarks are then set to be the categorial values, i.e. the possible grades.

import matplotlib.pyplot as plt

finalGrades = [-3, -3, 10, 2, 10, 0, 7, 7, 12, -3, 7, 0, 12, 12, 12 ,12, 12, 0, 0, 0, 4]
possibleGrades = [-3, 0, 2, 4, 7, 10, 12]
fin = [ possibleGrades.index(i) for i in finalGrades]
plt.hist(fin, bins=range(8), align="left")
plt.xticks(range(7), possibleGrades)

plt.title("Final Grades plot")
plt.xlabel("All possible grades")
plt.ylabel("Number of students")
plt.show()

Plotting in python3 (histogram)

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