Pandas Plotting (bar plot) non numerical data

Question

I have a series of data (climbing routes and their grades.)

pd.DataFrame({'route':['Easy route', 'The hard', 'Another route'], 'grade':['9a+', '6c', '7b+']})

route	Grade
Easy route	9a+
The Hard	6c
Another route	7b+

Grades are strings (non numerical), but can be easily converted to numerical data. Example:

def convert(grade):
    grades = ['6b+', '6c', '6c+', '7a', '7a+', '7b', '7b+', '7c', '7c+', '8a', '8a+', '8b', '8b+', '8c', '8c+', '9a', '9a+']
    return grades.index(grade)

Table like such:

pd.DataFrame({'route':['Easy route', 'The hard', 'Another route'], 'grade':[16, 6, 1]})
       route  grade
0     Easy route     16
1       The hard      6
2  Another route      1

Can be plotted with ease. What I want is a plot with the original grades, like this. What is the workaround for non numerical data?

   Easy route   *************************************
     The hard   ************
Another route   *****************************
                6a 6a+ 6b 6c 6c+ 7a 7a+ 7b 7b+ ... 9a+

Maybe plt.bar(x=df.route, height=df.grade)?

JohanC
– JohanC

2021-02-07 23:05:28 +00:00
Commented Feb 7, 2021 at 23:05 — JohanC
– JohanC, Commented Feb 7, 2021 at 23:05

Max · Accepted Answer · 2021-02-08 07:30:43Z

I suggest to first add a numeric representation of the grades in order to maintain the order when plotting. Afterwards, you can set the names of the grades manually as yticklabels:

from matplotlib import pyplot as plt
import pandas as pd
# to example
df = pd.DataFrame({'route':['Easy route', 'The hard', 'Another route'], 'grade':['9a+', '6c', '7b+']})

# original grades
grades = ['6b+', '6c', '6c+', '7a', '7a+', '7b', '7b+', '7c', '7c+', '8a', '8a+', '8b', '8b+', '8c', '8c+', '9a', '9a+']
# define function
def convert(grade,grades):
    return grades.index(grade)
# add numeric grades
df['grade_num'] = [convert(g,grades) for g in df['grade']]

# open figure
fig, ax = plt.subplots()
# ensure limits
ax.set_ylim([0,len(grades)])
ax.bar( x=df['route'], height=df['grade_num'] )
# ensure limits
ax.set_ylim([0,len(grades)])
# set ticks
ax.set_yticks( list(range(len(grades))) )
# set tick labels
ax.set_yticklabels( grades )

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